GPM from what most people say is crap.
I've had extremely good luck so far with the steering knuckles, rear wheel hubs, and shock towers. Now I did not have such good luck with the rear arms I had that were GPM. The rod's they gave to hold the rear arm onto the body are made so cheap and bent so badly its not funny. I actually thought something was wrong with my vision when I saw how my rear tires were sticking out at weird angles. Seeing both bent and warped in similar fashions I doubt it was a fluke with one. The ones I got from HPI on the front are still in perfect shape but to use HPI's I had to go back to nylon rear arms.

C hubs I thought about seeing I broke 2 when I broke the front knuckles, but once I replaced the knuckles with the aluminum I stopped breaking the C-hubs.

Can't you just bend the original chassis back into shape? If not I would just get another HPI chassis off ebay usually see them for around $20. The GPM one would probably crack from a hit that bent the stock one

If you can find it I've heard alot about billet aluminum but unfortunately I cannot find where to get MT2 parts made out of it. Maybe someone else can help here

If I had to do it over again I wouldn't have bought anything other than the knuckles from GPM the rest seems to have been a waste of money. Only benefit I actually received from the GPM rear shock tower was my homemade wing mount hooked onto it extremely easy other than that the HPI MT2 equivalent would have probably sufficed.(btw I have a extremely heavy modified MT1 that has 90%ish of its parts from a MT2)

So Knuckles get aluminum, rest get replacements from HPI IMO

it also sounds like you might need a new steering servo seeing you said its not responding too well. I'd try adjusting the D/R knob on the controller first though before replacing a servo