Looks like you have the same rear end I do that's meant for a Sun recumbent trike and cant be used to turn a standard bike into a trike without modification. It's designed to bolt up to a spot that mirrors it. A bolt goes into the each square hole on the frame and the the rear end. Two bolts within an inch isn't enough, there's just to much pressure.
The easiest solution without welding would be two flat bars that match the bolt pattern of the rear end but extended 4-6in. Then use a U bolt or something strong to clamp the extra 4-6in to the frame. You could also weld or bolt something to the basket bars on the rear end that extend and attach to the frame somehow. And if I remember correctly the GT2 is aluminum so it would be best to use both ideas somehow.