Hi, I am a fellow user who nearly panicked when I realized I couldn't use legacy addons or that it didn't detect gma files.

I was wondering what was wrong. Now, here's the thing. Extended Spawnmenu doesn't appear to detect GMA files. I understand some people here are downloading archived addons to use them again because of Ninendo. But, this doesn't detect those GMA files.

You have to extract the folder from that GMA file you downloaded, and go through the process of like your making an addon to upload to the workshop, which is possible with only one app. Notepad++ which I used to make the JSON and Bat files. BUT! You only need the JSON file, I think. Your not uploading it to the workshop... I don't advise it. It'd only get taken down if it's an addon like that, not to mention needing permission from the original poster.

Once you extract the addon, I think the folder along can function. ES can detect it and it will be found under the Legacy tab. I've tested it with a Hyperdimension addon, and it appears to work.

So you need to extract it, which I believe is done using in the Garrys Mod/Bin folder, this form below shows how it's done:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=431638110

Gmad seems to both extract and compile the gma file accordingly. So it doesn't appear to read GMA files. To me, it detects the folder once you extract the folder from the gma file. You can delete the GMA file after that, the folder with all those materials, maps, or whatever addon it is about, is all that matters.

I am not sure if the addon.json is necessary, but I did make it, and that required me to use notepad++ to just copy paste the:
{
"title" : "My Server Content",
"type" : "ServerContent",
"tags" : [ "roleplay", "realism" ],
"ignore" :
[
"*.psd",
"*.vcproj",
"*.svn*"
]
}
From the wiki into, but I don't believe this is necessary. In fact, I think all you need is the folder.

I suspect some people here may be just dropping in the GMA file and thinking that will work. I know I did.

No, it does not. Not sure if the creator mentioned it, but you need to extract the folder from the gma file. Which is usually done by first you place the gma files into the addon folder of Garry's Mod. Then you open up another folder as to go to the Bin folder of Garry's mod and then one by one insert the GMA files into Gmad.exe, where it will extract the folders and place them into where that gma file is located, which most tutorial videos I saw is in the addon folder. IDK what happens if it's not located in the addon folder. After the folder is extracted, that gma file is no longer needed. You can get rid of it to save space on your PC, or whatever.

This is what worked for me. I cannot promise it won't work for you, but this is how I was able to get Legacy Addons to work. You need the folder, not the gma file. It searches for those folders, as if you were creating an addon to upload to the workshop according to the wiki. Or I suppose this was how addons used to be like.

Again, this worked for me. IDK if it'll work for everyone, but this is why it didn't work for me. Otherwise, this addon works pretty good. I've had no issues other than "Slightly Longer" loading time than usual. Then again, slightly longer loading time is a decent trade off to being able to continue my Gmod animations on YT.