Recommendations

I have learned Html and CSS. But I am wanting to make forums for my gmod server what all languages would I have to know to do so.

You need to learn PHP, for sure

for a Gmod server, it depends on how much integration you want I recommend a good knowledge of HTML, CSS, JS, PHP, MySQL and that's not including style libraries and API's which you h=would need. Ill include some cool forum software which you sometimes have to pay for but are a lot easier than making your own forum software. Also there are things such as PHPbb and myBB but they're not the best

also if you're willing to spend some money then you can go for the best - https://xenforo.com/

https://noclip.me/ - a nice free one haven't used it personally but looks good from what communities have used it

https://www.enjin.com/forums - one of the OG's but not the best

https://www.mistforums.com/ - this is dedicated for Gmod and is easy to use I recommend this for first-time forum setup