So after looking at the code and trying to compile it, a few things:
1. You're probably going to want to change "<gmg/pets>" to just "<pets>" and pop the pets.inc contents into your "include" folder
2. You're gonna want to add this to the top of pets.sp:
PHP Code:
#include <sourcemod>
#include <sdktools>
#include <tf2>
#include <tf2_stocks>
#include <sdkhooks>
Otherwise you'll have huge error spam when you try to compile
3. Once you've done 1 and 2, the only error left will be the lack of a TraceEntityFilterPlayer function which you can find in many plugins.
I haven't fully compiled this or added in a command to spawn a pet and test it, but it looks promising.