According to Wikipedia:
"Despite being a standalone game, Garry's Mod requires the user to own at least one game on Steam based on the Source engine, such as Half-Life 2, Counter-Strike: Source, Portal, or Team Fortress 2".
Since (again according to Wikipedia) each of those games has generally the same system requirements as Garry's Mod, it seems reasonable to assume that if your laptop will run (for example) Half-Life 2, then it will run Garry's Mod.
Chris