Should I modify the end trusses to accomodate a gable vent?
The only circumstance under which I'd even remotely consider modifying a truss is if I had an engineer's stamp. 

In this specific case, you're thinking about the opening as a simple downward carrying load, and I don't think either of us are qualified to make that assumption. (Trusses probably have side and up loads that aren't obvious to mere mortals.)

I'm not sure why you wouldn't just have an asymmetric vent, or split the vent between two smaller symmetrical vents.