Does anyone know if there are any decent Mechwarrior-type shooters on Gamecube?

Domestic or import will do. (Freeloader rules! )

[RL123]

Gotcha Force (Capcom) is very good, although I haven't had a chance to buy it yet.

If you wait a while until its release, you can buy Custom Robo (by Nintendo) which is supposed to be a fantastic game. I'm sure there are a few more choices but, IMO, those two are your best options.