On the CachyOS box (RTX 4060, NVIDIA proprietary, Xwayland :0), native Linux Garry’s Mod hangs forever on the loading screen with a full progress bar. Hosting a local server it aborts ~15s in (SIGABRT, ~90MB core, right after Game.so loaded for "My Garry's Mod Server").
Diagnosis: gdb attached to the live process shows the main thread parked on one instruction inside shaderapidx9.so -> materialsystem.so -> stdshader_dx9.so -> client.so — the threaded material/shader queue dead-waiting during world load. Console shows Could not load program cache file glbaseshaders.cfg, Could not find base GL shader cache file, and every menu background Failed to create material for background -> No main menu backgrounds found! (grey main menu).
Fix that worked: mat_queue_mode "0" (single-threaded material queue). Written to garrysmod/cfg/autoexec.cfg; map then loads normally. Alternative if it recurs: run the game under Proton instead of the native build.
Side note: the game is on the USB HDD /mnt/External/SteamLibrary; host has no 32-bit libXi.so.6 (lib32-libxi not installed), so launching hl2.sh outside Steam dies with Couldn't load garrysmod/bin/client.so — libXi.so.6: cannot open shared object file — Steam’s runtime container masks that.