Lua Panic: Not enough memory

The "Lua Panic - Not Enough Memory" error is a sign of Garry's Mod running out of memory (RAM) to work with.

This means one of the following:

1. Your PC does not meet the minimum requirements to play the game. You need at least 2-3GB of free RAM to play the game, or at least 4-8GB RAM installed as mentioned in the System Requirements sections on the Steam Store page for Garry's Mod.
2. You are trying to join content heavy and unoptimized servers. DarkRP and other Roleplay servers are notorious for this. Please note that since all of the custom content on those servers is not created by Facepunch, there's nothing Facepunch can do to "optimize" the content. This is entirely up to the server administration and the content creators to do.

To solve this sort of crashing, you have the following options:

1. Upgrade your RAM if you do not need the minimum requirements to run Garry's Mod.
2. Reduce your graphics settings, especially Texture Detail.
3. Uninstall or disable unused addons. 
4. Restart your game if you are joining a server after just playing on another server or in singleplayer.
5. Try the x64-86 Beta branch for Garry's Mod in Steam.
   • To change Beta branches for Garry's Mod, do the following:
     1. Open your Steam Library
     2. Right click on Garry's Mod and select Properties
     3. Switch to the "Betas" tab
     4. Select the preferred Beta version from the dropdown list
        • Please note that you do not need to enter any "keys" or "codes" to access Beta versions of Garry's Mod
        • There's a known issue with the dropdown menu where it simply will not appear. In that case you can use your arrow keys to manually scroll through the betas.
        • If at any point you'd like to restore your game to the default beta, simply select "NONE - Opt out of all beta programs"

Engine Error: Failed to lock vertex/index buffer

"Failed to lock vertex buffer" and "Failed to lock index buffer" are also memory related crashes but this time have to do with VRAM, the memory capacity of your graphics card.

This can be caused by your graphics card not meeting the minimum or recommended system requirements for Garry's Mod. You can find the system requirements for Garry's Mod on its Steam Store page. Unfortunately you can't really "fix" the problem in this case without upgrading your PC to meet the minimum requirements.

It can also be caused by joining very content heavy servers with insufficient hardware to play on them. DarkRP and other Roleplay servers are again notorious for this. Please note that since all of the custom content on those servers is not created by Facepunch, there's nothing Facepunch can do to "optimize" the content. This is entirely up to the server administration and the content creators to do.

You can try to mitigate this error by reducing your graphics options and game resolution. Reducing addon count is also not a bad idea.

Please note that integrated graphics accelerators (APUs or iGPUs) will not be sufficient to run Garry's Mod for their lack of dedicated video memory (VRAM) and therefore are not officially supported.