SM does support Garry's Mod.
Did you install any other server or SM plugins beside just MM:S, SM, and the base plugins that come with SM?
Edit:
FWIW
Code:
version
Protocol version 15
Exe version 1.0.13.0 (garrysmod)
Exe build: 12:08:12 Feb 21 2011 (4426) (4000)
sm version
SourceMod Version Information:
SourceMod Version: 1.3.6
SourcePawn Engine: SourcePawn 1.1, jit-x86 (build 1.3.6)
SourcePawn API: v1 = 4, v2 = 3
Compiled on: Oct 30 2010 19:26:52
Build ID: 3065:8036343199ca
http://www.sourcemod.net/