@BowmanWatson
Ok, so the first step is to check your addons, if you have a tonne of addons subscribed to then your gmod may not like one of your addons with the server addons.
Next step: Verify the integrity of your game files, open up steam, right-click Garry's Mod, properties, click local files then verify.
If failed above ^ then
Next step: Go back into your properties mentioned above, this time instead of local files, hit betas, click the drop-down menu and choose the x86-64 beta then run your gmod and try booting into the server.
If unsuccessful above ^ then
Next Step: Sadly we need to take the long route here, back into your gmod properties, betas, opt-out of all betas (Should say NONE), then open up your local gmod directory, navigate to Garrys Mod/garrysmod/ then find and ctrl-left-click each of these folders
addons
materials
resources
models
ALL download folders, you will either have 1, 2 or 3 download folders depending on how many game modes you have played, regardless of how many you have just click all download folders.
Particles
And Cache.
Once highlighted you can go ahead and delete these folders. These are NOT core folders and if some servers through their FastDL forcefully download files onto the client then they will be stored in these folders and you could have a LOT of space taken up on your gmod because of this. MAKE SURE YOUR GARRY'S MOD IS CLOSED WHEN DELETING THESE. If your Garry's Mod is open then it will just forcefully download these things again which is NOT what you want.
Once you have deleted these, open up steam, right-click gmod again, properties, local files then verify all over again and this should be successful because your gmod will ONLY verify your core files, for clarification you core files are what you need to actually run gmod so your verify will be successful and will complete the process, now once verified, open up your gmod, DO NOT subscribe to anything on the workshop, THEN try and log in to Revival HaloRP.
Now if that is unsuccessful above ^ then
Last Step: Time of an upgrade in Hardware sadly.
Any questions feel free to hmu in PM's and I'll help you out!
Hopefully, these steps fix your issue.