I know you said you were looking for a gas trimmers, but here's 2 cents
about the Toro Model 51556 , 7" Cordless Trimmer.

This is the first trimmer I've owned (having graduated from my parents'
manual 'wheel on a stick' trimmer. I don't know if this trimmer maybe a
little "wuss" for your purposes (it has the small diameter string I've
seen, .050"), but it works fine for me, usually cuts everything in one
pass. The only problems I've found is the trimmer length isn't adjustable
(I'm a little too tall for it) and if you are out of shape, the battery
gets heavy.