For any blowgun frog gig,four or six pound test line is heavy enough. The biggest bullfrog on record was just over two pounds.
There is a way to use a line on almost any blowgun. It is called a T spindle, and I designed it several years ago. You don't have a reel, and you have to rewind it every time you use it by hand, but you can make one with a pocket knife out of wood. Sort of a "survival" device. Very effective for short range fishing. I will see if I can throw one together and post a couple of pictures soon. It is pretty obvious, but no one seems to have come up with it before.