The GMT355 chassis was co-developed between Isuzu and General Motors, but for the most part, it was an Insuzu design all the way through. General Motors, however, had the exclusive rights to sell it/use it first in the US. The only variant on the chassis is the GMT345 version, which sits beneath the Hummer H3 and H3T. Presumably there will be a new variant of this chassis, yet to be named, which will underpin the H4.