Can you use garrysmod with counter-strike 1.6?

No you cannot because Counter Strike 1.6 runs on the original counter strike engine. Every game on garrysmod must run from the Source Engine or any game ran off of HL2.

From my memory, These are the requirments. You must have ONE for gmod to work.

Garrys Mod*Required

Half Life 2 *You need this for basically everything in Garrys mod

Half Life 2 Episode One

Half Life 2 Episode Two

Counter Strike Source

Team Fortress 2

Half Life Source

Day of Defeat Source