Its not overly flexible is just far less brittle as the other West systems 105 and other epoxy's. When I was stress testing it actually had a great breaking strength the standard epoxy's.
Mix up some equal amounts of both types in the small plastic mixing cup's and let it cure for 24 hrs. The standard West Systems 105 will just about fall out of the mixing cup. But not the G Flex you'll have to destroy the mixing Cup to get to break the bond.
They also offer the GFlex in tubes that are pre-thickened. I'll tell another great adhesive for flexibility it's E6000 it can be found in craft stores and glues almost anything. Similar to silicone but way tougher. The only glue I've found that can glue the sullivan fuel tanks together. When you pull it off of something it keeps stretching untill it finally releases most cases in one piece.