I use Gelish gel polish system and find it gives more strength than a normal polish. Also the Gelish system has a Structure Gel that you could add to the service for clients with weak nails to give more strength.
However you need to use the whole system.
If you are looking for a gel topcoat to go over a normal polish then that is a bit more difficult to answer, as gel topcoats need either a rough surface or an inhibition layer to adhere properly.
I know some geeks on here have tried this with some uv topcoats possibly Glaze & go but the polish has to be totally dry/set before you can apply it.
If you do a search on the site you may find some old threads discussing this.
You may be lucky and get a reply today, but I suspect it will be a little quieter on here as there is a big Trade Show on in the UK and loads of geeks will be there !!