Many advanced packs (especially for Wiremod or ACF) are hosted on GitHub. You download the master branch and extract it to garrysmod/addons/ .

A raw GMod screenshot often looks dated by modern standards. However,addon packs that include can transform the 2004 Source Engine look into something photorealistic.

If you want GMod to look and feel like a modern tactical shooter, these community-favourite addons are non-negotiable: Top 10 Gmod Addons Of ALL Time