Hi Pang !
I have both a ranger and a gadgeteer in my (first) party. And I love both
The ranger because of his ranged criticals "A magnificent kill i would say"
and the gadgeeter because of the many strange items you can buildt.
So I think it is a question of preference.
The ranger makes more damage, but the gadgeeter can use his items...
And to your last question: at my first try I changed a rogue into a gadgeeter and he got his omnigun immediatly "from the goods"... It was just there
Hope that helps
Silaya