You need to clarify your terminology, both in your head and to us, as theres an element of mixing your mediums in there!
Gel extensions are created with either Hard Gel or Soak Off gel - over a tip or sculpted over a form.
Gel polish is not strong enough to use to create extensions. Unless you understand the terminology you will get confused as many newbie techs are classing 'gel polish' as gel nails. They are saying 'I do gel nails can you recommend a brand....but actually they mean they are doing Gel Polish' - Gel nails is generally understood within the industry to mean extesions created with hard/soak off gel.
Gel paint are something completely different and nothing to do with nails, they're a craft item that can be used for nail art. ETA - I googled Gel Paints, looks like a supplier is producing a range of highly pigmented gels for nail art - I've not seen them before, but I think someone else has also replied saying they can be used for nail art.
Its not normally necessary to use Gel Polish over Gel extensions (or L&P) as normal polish adheres particularly well and rarely chips or peels. Gel polish is expensive, sometimes 4 times the price of a normal polish so you would need to factor the extra costs into providing clients with Gel Polish over Gel extensions.
There are some Hard Gel products available that are highly pigmented and 'colour' gels which can be used for creating extensions but they are not common and quite expensive as normal polish does the job quicker and easier. Plus you can change a polish colour without having to remove the entire extension if you had used coloured gel.
You can also use normal crafting paints (acrylic paints) for nail art over Gel extensions/polish/gel polish/L&P if you wish.
Hope this helps clarify
