No responses yet so I'll throw my input here. Not the same unit but very similar tow vehicle. Look at my signature- gas engine like yours.
Recently went from carrying a very heavy truck camper to pulling a short 5th wheel.
Our new-to-us 5th is 7000# empty, so that would be a ton less than a GD 303. We are in the New Hampshire White Mountains and while the truck doesn't pull like a diesel, it doesn't struggle. It gets about 9 or so mpg in our "hills". The 6 speed trans is a big improvement over the older 4 speed.
On the flats to rolling terrain you should be good. Hopefully others will add their input. But the diesel folks might have different opinions.
John & Cathy
'12 Chevy 2500HD CC 4x4 sb
'16 Cougar 25RKS w/ Andersen rail mount
'13 Eagle Cap 850 (sold). B4 that a few other TCs and a TT