What does this error mean?
To play on the SSTRP.net server you'll need to be using the 64-bit version of Garry's Mod. This is because we rely on a number of technologies in the x86-64 branch that aren't in the normal branch of the game. Aside from that, you will also see a significant performance increase using the newer branch.
How can I fix this?
Step 1:
Navigate to your Steam library, right click Garry's Mod, and click "Properties".
Step 2:
Click the "Betas" tab along the left.
Step 3:
Select x86-64 - Chromium + 64 bit binaries.
Step 4:
Exit out of the Garry's Mod properties screen and relaunch your game. You will now be using the x86-64 version of Garry's mod