A gel that is, say, 7.4mm being forced down a 7.0mm barrel will have to become oblong to get down the barrel… then, with X amount of compressed air pushing it, the back of the gel will possibly become concave, pushing the gel out towards the barrel, causing much friction.
Engineers on here are welcome to correct me, but I think the gel should be about .2 of a mm smaller than the id of the barrel… so the ball has a little clearance, but when the compressed air pushes it, the gel expands to make a reasonable seal on the barrel.
If the gel is too small, like a 7mm gel in a 7.5mm barrel, the air will just go around the ball and it will not go far, or maybe fall out of the end of the barrel … DoH!!
It is not too difficult to change out barrels… it might be an idea to measure your gels and change barrels to suit…
BTW, gel blaster barrels and gels make awesome pea shooters