we found the gdiapers not to be the best for overnights, so we use a pocket dipe stuffed to the gills.
For daytimes we stuff the g covers with a prefold. Hemp doesn't absorb as fast as other materials so it could be that you have some 'loose' liquid in the liner that doesn't absorb so it wicks out the sides. Also if whatever you stuff it with is too thin it can move around in the liner and not absorb at all, that's what the flushies did when we used them.
This may sound like a really stupid question so please don't think me unkind, but I have to ask -- are you using the nylon snap in liner with the gdiaper cover? That part is essential and without it you would get some pretty quick wetness and I can imagine it would be frustrating!
Also, for us, cloth had a pretty big learning curve. It took us a couple days to even figure out a routine, not to mention get the leaks under control, have confidence, it will work out!!
Also if your onesies are a pain and you don't have just plain t shirts you can cut off the 'tails' and get a tshirt.