This is a rough guide to share info and will contain spoilers.
The codes to the first secret gates is in the tablets after it's all said and done.
For the desert tablets the third tablet is both directions through the maze
AND the 1st gate code.

Still working on the 2nd gates.

Tablet 1

In 1-2: "Shortcut through supplies" there's a sign saying dead end. If you go to the dead end you'll find a loooooot of boxes. There's a hidden path you can navigate through to get to the tablet!

Tablet 2

In 1-4: "Bluffs" there's another sign saying dead end. Go to the dead end and navigate the death trap to find another tablet!

Tablet 3

In 1-A: "Fences" There's a spot you can get around the fence and go left. Do so and it's tablet time!?

Tablet 4

In 1-C: "Unmistakable Accents" at the very start if you look carefully you can see the first tablt so... uh... touch it. Touch it softly.


Now for the other half of each tablet.

Tablet 1 other half
Leap attack here

Leap attack in 1-1 to reach the second half of this tablet.

Tablet 2 other half
Hit "enter level" three times here. Might be another button.
Then go through 1-D to the tablet

Tablet 3 other half
Go here and hit up, up, left to unlock a level
Tablet is to the left.

Tablet 4 other half
There's a hidden box down from this spot. Break it, head right through the trees.

In 1-5: "Royal Gardens" there's a really well hidden path on the east side where you need to break hard to see boxes that lead to a hidden path eastwards and the third tablet!

CONGRATULATIONS! First worlds tablets are done.

These are much more complex... PREPARE YOURSELF!
Work in progress

Tablet 1
Below a grass arrow pointing down in 2-4:

Go down here and it's Stone time.

Tablet 2
in 2-C (Credit to OVALLODA) There's a sign saying dead so go there anyways.
Break boxes and reeeeeeeeead.


Tablet 3
Go through gunners cave

Tablet 4

In 2-B to the right in the forest path is a hidden path blocked by a box. Break it and follow it to a sneaky stone.

Other half of these tablets.

Tablet 1 part 2
Go here and hit the button with no real use, R1 on xbox360 controller.
Opens a level where there's a tablet

Tablet 2 part 2... Sigh...
Go to 2-3
The tree it refers to is to the right
IT'S A LONELY TREE! So go west
Hidden box, beat that box and go west
A bunch of hints here, too lazy to list them.
Go here and do EXACTLY THIS: Hit "enter level" button, right, left, right, camp button.

New place'll open up and youn can stare at stones.

Tablet 3 part 2
In New Path Old Routines howl, as tank, at the moon
Return to the overworld and go right and keep going right to unlock a secret area that's kinda tough.
Go south here in the moon level and keep going south. Stone tablets at the end.


Tablet 4 part 2


In 2-6 the hedge maze has a secret path with A BIGGER MAZE then stoney mcstoneface

GOOD JOB!

There's only one/two tablets here

Tablet 1
Beat Charon to Find the gate further down.
At the VERY bottom next to the 1st gate computer
Mentions symmetry.
SO
Directly opposite or it, to the right, is an invisible tablet with another code.

The code is symmetrical to these two together.

bTAC-CATb.

These are weird. You mainly just need to read them and do some stuff.

Tablet 1
at the bottom of 3-2 is a tablet. You need to break the stones to the left of it.

Tablet 2
in 3-3 theres a sign saying dead end south so we need to run to it and dead end it up. Don't die to bullets.

Tablet 3 and 4
These two kinda just show up as you figure out the puzzles. You'll see.

Other Halves
Tablet 1 part 2

In 3-A you can break the stones to the right and you'll be able to walk up this wall after, if you try. Then there's a really annoying place with invisible death walls and if you go to the top left there's a treasure. You'll have to exit through the north and it activats the second half of the tablet when you exit.

Tablet 2 part 2
Go here and hit the pause button for a secret level to run through and exiting activatrs the second part of the tablet.

Tablet 3 part 2
THIS IS ALSO THE COMPUTER GATE CODE! Go to mirage temple and use this code as directions; Left, up, right, up, up, right, left, up.

Tablet 4 part 2
See the little circles? Go to the 4 way in mirage temple and wait in that spot until the cat shows you where to go. So go to the four way, wait in the top left, follow the cats directions, then wait in the bottom left, follow cats arrow, top right, follow cat arrow, bottom left, follow cat arrow, top left, follow cat arrow. I'll add more screen shots if I need to.

Congratulations?

3 of the 4 main levels has dead ends leading to the stones. Add screens later.


The Styx level has darker colored boxes near the end that you can walk through to find the secret area leading to that stone.

The solution for the gate: PAY ATTENTION to which stars the stone shows as the bright ones.
Follow the ram north to the swan, follow swans long arm to the crabby patty, follow the crab to the lion. Go opposite the bright star on the lion.

The stars will start to be funny and move. Try to align them and it'll vaguely begin to look like the main 4 shapes in groups of three stars.
So the code is T C A B
*
C A B T
Or: TCAB*CABT

Lazy so this'll be text for a while.

Menoetius 2nd gate looks like music. Pretend each one is a number from 1 to 4.
The stars on the bottom are one, top are four.
Thus: 1 2 4 3 1 2 1
When you play an instrument in camp after a certain point itll have a number of asterisks next to each note you play. So play 1243121 and itll give you a code: ATBA*CBAB

Coeus 2nd gate
Turn on hearing impaired mode
Set radio in camp to "Static"
Pick Meg
Slow down time with special
it'll show dots; 3 4 2 1 1 2 4 3
Thus the code is BTCA*ACTB

Styx Second Gate
Read the first gates journal
Use these as directional inputs to put in at a MISSION FAILED screen to get the code.

Lelantos 2nd gate
The video is the trailer, although, I recommend going to one where you can pause it.
It'll give you three lines of codes
In Khomli journal it'll show which of these to use
Line 'em up
You'll get a code: TACT*ABTB

Chronus 2nd gate
the VERY beginning is the TITLE SCREEN and no "breaks" means no demos. So keep hitting left or right and, eventually, in the top right, some stars will appear with the second gate code.
The stars'll end up being: bTbA*ACTA

This one's actually pretty simple once you know it

So the code is in the order of the statues

Leader, Loyal, Inference, Intuition
Triss is leader, Ano
Alex is Loyal, Ceyo
Erin is Intellect/Inference, Go
Meg is weird but intuitive, Toh

So 1st half is ACbT

Second half is GO CAT statue
Go is b and cat is CAT

So...
Kings gate code is

ACbT*bCAT

Pause once in-game.

Press left,left,right,right,up,down,up,down,right,left
It's based on the number on Lelantos Trophy.

You might want to add some in the comments in case I don't come back to the game...

1st Meno gate: AbCT*bCTA
1st Foloi gate:CTAb*bTCT
1st Lela gate:
1st Styx gate: Noooot going to kill Charon in NG+
1st Chronus gate: TCAb*CAbT