Here is a useful chart for CPU's from gamers nexus, CPU dependency. Better CPU = better frames on gmod and other games. the 6700xt is more than enough for gmod as a GPU.
Having an excellent CPU with a lot of cache such as the rx 7800x3d (Ryzon "3D" series is shown to provide more FPS as seen in @HuskyD0G benchmark above). However, this is obviously not ideal as it will be quite expensive.
Sometimes, having too many addons can also cause some weird glitches and can slightly affect frames (for me at least), so I would also recommend to ONLY have the perpheads addons enabled.
Make sure multi-core rendering is enabled and also try not to run as much applications in the background that uses a lot of CPU and RAM (Chrome, Discord etc..)
That is about all I can think of, GMOD is quite an old game and the fact that the game is still running 45 - 60fps with 128 players is a miracle considering the frames we used to get back in the day with only 60 players.
Alongside what Megasaw has suggested, put -high in your launch commandline or change the priority of gmod/hl2.exe to high in task manager (you'll have to do this each time unless you do the commandline way), you may notice a increase big or small
Alongside what Megasaw has suggested, put -high in your launch commandline or change the priority of gmod/hl2.exe to high in task manager (you'll have to do this each time unless you do the commandline way), you may notice a increase big or small