I haven't tried the GU gels personally, but I've never had a problem with the SIS Go gels and stomach cramps. As Harry says the solution will likely be unique to you so try loads of things: different brands of gel, water bottle, hydrate more on the bike and use less gels on the run....
I'd be really interested to see what you find out?
similarly to a lot of other comments above, i find the standard gels too thick, particularly for running with. the hydro/go type seem to be more usable (high 5, powergel and sis do them). on the bike i don't mind so much but still tend to take the gels with water.
i really like soreen for cycling - sliced loaf with butter put back in its packet before stuffing it into a jersey pocket. i'm a little mindful of how much fibre there is in a loaf, though!
for a half iron i've used energy bars/soreen on the bike and switched to gels that i know i can stomach on the run. i usually carry a couple of gels for the bike in case i get fed up with the taste of the bars or want a little hit just before i come into t2.
bridgtown cona testa tri team - proudly finishing at the back
most disturbing in lycra award, 2010, 2012, 2013
For bike nutrition you cant beat my ultimate flapjacks...