I cant afford photo shop and substance painter 
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Basically most people use this for UI:
Figma:
Its pretty nifty, has more features than Roblox Studio UI, it has the standard and complex union operations but for UI elements, add gradients to basically anything, has prototyping, has built in auto-layout, etc and more
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figma and gimp thats all the answers you need
like 2jammers said, figma and gimp are pretty good. paint dot net is also a very nice app for textures and such (the one downloaded from the website is free)
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thank you to anyone who said figma I am going to learn about it now I can only mark one post for the solution unfortunately