Technically we should all be using Adobe Illustrator when designing shirts for Merch by Amazon. However, I know many of you use Gimp because it is free. There is an alternative app I recommend besides Gimp and the price is just as good. It’s free!
Pixlr is a robust photo editor for both Windows and Apple that you can use for free on your browser. I can’t compare Pixlr to Gimp because I never used Gimp but compared Pixlr compared to Photoshop is just as good. You can even purchase a desktop version for a fraction of the price compared to Photoshop. You can purchase a monthly subscription for a measly $1.99 or an annual subscription for just $14.99.