Recently, Steam has being doing various maintenance, causing their server loads to go up and down. In return, this has effected client downloads upon joining servers. Unfortunately, there are no server configurations we can change to fix this until Garry's Mod releases an official update. Rubat/robotboy655 (Garry's Mod head developer) addresses this issue here: https://github.com/Facepunch/garrysmod-issues/issues/5409#issuecomment-1509884225.
However, Rubat has informed everyone to switch to the "dev" version of Garry's Mod to temporarily fix the issue. Below is a step by step guide to do this.
Step 1: locate the game within your library tab on Steam
Step 2: Right-click the game and select "Properties"
Step 3: Select the "Betas" tab
Step 4: Click on the drop down arrow
Step 5: Select the "dev - Development Branch" option
Afterwards, your game should do a short update and then you're all set to go. Open the game and join the server, content should be downloaded. In some cases, depending on your steam server and its downtime, other connection/download issues may occur. If so, wait awhile and try joining the server again.
Hope this helps!