The only thing you can use to remove gel is acetone. Make sure you are using pure acetone. The stuff they sell at convenient stores tend to be weaker.
First, file off the top layer of the gel. Be careful, most nail techs will file through the coat into the actual nail, damaging it. Then wrap with acetone and foil. Make sure you are letting it soak for a good 10 mins. Using cuticle pusher, you can then push the gel off of the nail. Usually the tip and the middle section of the nail is the hardest to remove because nt's tend to buff these areas too good. If there are stubborn spots, use the nail file to file it off gently.
Gelish isn't a good brand. It tends to stick too strongly to the nail plate, making it hard to remove. If you scrap too hard youll damage the nails. That is why I recommend OPI for gels. It will swell up and break apart.