Ive used Gelish over tips several times, both to lenghten a broken nail to fit the others, or for full set enhancements. I usually only use Gelish to do this either when the nails are not meant to last long and the client is not going to come in for a rebalance, so they can be easily removed or on super chewed up nails that the hard gel has problems sticking to.
I did one girls nails today with Gelish over tips. She is trying to stop biting her nails but had a terrible setback the other day and wanted something to cover it up. I used Gelish because then in 2-3 weeks time I can soak them off in 20 min and apply normal Gelish since her nails will have recovered some in that time
Contrary to some people belief on here the Gelish structure is actually pretty darn strong
I myself have worn ca 1 cm enhancements with tips and structure for 2 weeks and none had broken or looked bad in any way.
Just make sure since the gel is very thin that you are able to make a good apex. I usually brush a medium layer of it over the entire nail, then take a bigger bead on my brush and put it in the apex are and just even it out a tiny little bit with my brush before putting it in the lamp. It will self-settle very fast.
Hope this helps