VFR,
It very much is!
I am pretty sure that this gliderdriver did not think: 'Let's bank right and I have right of way over the power driver so I will be fine!'
More than likely he did not see you. Especially from the angle you describe it will be difficult to see anything in which is beyond your leading edge.
If he did indeed come from your overhead initially it makes it a bit more difficult to appreciate that he was not aware of you but it does happen to all of us that we miss something.
A salutory lesson in how little we actually see despite keeping a sharp look out can be had by flying a machine with TCAS in the open FIR. Despite 'knowing' (as in bearing and distance) where to look you it can be a devil's job to spot the bogey!.
And before any of your wisecracks pipes up; I do have 20/20 vision, sometimes even without hindsight!
FD