is it possible to strengthen the gt engine so that it is safe to go above 6800
and if so, what needs to be changed
To answer your question directly, you might have to look into purchasing upgraded valve springs as valve float is the most common problem when increasing the redline.
And as the others said, to take advantage of the increased revs, you will have to considerably increase the volumetric efficiency at higher revs, I.E. cams.