I'm not sure what the third pet would be on that team.
Mechanical = Strong against beasts (Likely bears in PvP, which you want to stomp before they stomp you); weak against elementals (Ragnaros, Phoenix hatchling)
Undead = Strong against Dragonkin (Lots of dragonkin being used in pvp), weak against critters (Not often used) and Aquatic get a power bonus against it I think (Sometimes but not often used)
Undead is a pretty strong type for PvP for those reasons. You might want to make up for the mechanical's elemental disadvantage by adding a critter (Strong against elemental, weak against beast)
What would be the best critter to use? A healer?
What other good comps can you think of that would work well with the mech gnome?