Garry's Mod is an old game. By default, it's a 32-bit app.
This means (among other things) it's limited on how much of your system's RAM it can use (~4gb).

Memory isn't usually a huge concern for Garry's Mod (it was made to only use 4gb), but the x86-64 branch comes with some other benefits:

• Access to experimental multi-threading features
  
• Major update to the embedded Chromium browser (Chrome 18 [2012] -> Chrome 86 [2020])
  
• Various performance enhancements, more efficient use of system resources, etc.

In theory, the x86-64 branch should perform better. Your frame rate should be higher, your load times should be lower, and your overall experience should be better.

In practice there are a few concerns you should consider:

The updated Chromium version allows your game to make use of the latest HTTP security features, but it's also harder to protect you from malicious addons that use the browser's extra capabilities. Web browsers are powerful, and the Facepunch team is concerned that the newer version of Chromium could open users up to obscure attacks and/or privacy concerns.

Some older computers won't be able to take advantage of the performance enhancements that x86-64 provides. Most of the time in these cases, the user notices no difference. Sometimes, though, this leads to stability problems.

Garry's Mod uses Lua for its non-source game code, we all know this. What you might not know is that Garry's Mod uses a special flavor of Lua called "Lua Just in Time". Very simply, it's supposed to be fast Lua. In general, it is impressively fast. However, on the x86-64 branch, LuaJIT appears to perform significantly worse in some cases - which may actually reduce overall performance. This seems to depend a lot on which addons you use most.

This is still technically an experimental branch. This means Facepunch can't guarantee that your security, performance, stability, or experience will be optimal.

• It might be generally better
  
• You can use cooler addons and see cooler loading screens / motds
  
• Get big frames
  
• You probably won't be subject to a catastrophic 0-day attack

Really, the biggest concern in my eyes (and purportedly, Facepunch's too) is the risk of falling victim to a new exploit.

These exploits could range in severity. Some exploits may allow other players to crash your game. Or, if a malicious player found a really bad exploit, they may be able to gain access to your whole PC.

There's no way of knowing how safe it actually is. That's the tradeoff.

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Yes! Servers can indeed use the new branch.

How do I use it on my server?
Because everyone has a different setup for their servers, it'd be very difficult for me to explain every way to do this. I'm afraid you'll need to use some google-fu for this.

I will, however, offer some advice for some common scenarios:

If you rent a server from a company:

• You should try looking through your server panel for "branch" or "betas" settings. If you can find these options, you should be able to figure out how to configure it.
  
• If you can't find anything in your interface, try submitting a support ticket asking if they can help you

If you self-host

• SteamCMD has a -beta flag you can use during install. You can install a new server instance and move over all of your files.
  
• LGSM has a simple config option that lets you set your branch. After updating it to x86-64, you can run ./gmodserver install again to have it acquire all of the new files you need.

Well.. that depends.

I've run our servers on the 64-bit branch for a couple of years now. In my experience, we haven't had any notable performance boost. We have, however, had a lot more engine crashes.

It's also very difficult to get third-party binary modules for 64-bit, because most developers only compile for the 32-bit version of the game.

Ultimately, I don't recommend it, unless you plan on sending crash dumps over to Rubat on a daily basis :)

I'd love your help!

Please feel free to leave a comment or reach out to me directly with corrections, enhancements, suggestions, etc. I want this guide to be as helpful as possible!