You can find 3 Stargates around the world. Here are Stargates and codes for every of them (I will explain codes later):

Costa Sud (40° 45.496' N i 5° 44.620' E) - 032310
Feno (40° 42.416' N i 5° 43.608' E) - 122112
Trio (40° 42.266' N i 5° 35.206' E) - 103013

Near every Stargate that looks like rocket silo, you can find 4 glowing working buttons.

0 is for 1st button from the left.
1 is for 2nd button.
2 is for 3rd button.
3 is for 4th button.

You must press them in correct order and Stargate gonna open for some seconds.
After jumping into it you gonna see short animation and you will jump out of the other Stargate.

Have fun!