Yes, of course.
If they're weapons or NPCs, you must use the Crowbar tool to decompile the models and edit the .qc file for it to work properly in Gmod.
When you download the Half-Life 2 addon, you must open the addon's .vpk file to extract the necessary files, edit them, and then convert them for Gmod.
There are actually several steps involved, but look them up on the Valve wiki or watch tutorials on YouTube.