is learning Photoshop necessary for making 2D games in Unity?

I use GIMP as an alternative to Photoshop because it is free and does everything I need it to.

No but if you want to make attractive then it needs it's only for design not for coding

Game devs use all tools. And some more besides. Photoshop is used for painting, and anything which requires precise pixel placement, stroke based painting, filtering and so on. ... But the dirt and scratches, burn marks, and compositing of layers would be done in a pixel art package.

if you want to make pixel arts you can try Aseprite.

is there any alternative to Photoshop? @Rajababu Shah