GIMP (GNU Image Manipulation Program) is a free and open-source raster graphics editor used for image retouching and editing, free-form drawing, converting between different image formats, and more specialized tasks. GIMP allows to create images with tools for airbrushing, penciling, cloning, and creating gradients. Power users can create their own brushes and patterns to use later. It also allows users to manipulate images imported into the program, further allowing to crop, add text, resize, and create nested layers.
Krita is a free-software and an open-source raster graphics editor, designed primarily for digital painting and animation purposes. It features a low-distract UI, high-quality OpenGL accelerated canvas, color management support, advanced brush engine, non-destructive layers and masks, group-based layer management, vector artwork support and switchable customization profiles. Krita is a FREE and open source painting tool designed for concept artists, illustrators, matte and texture artists, and the VFX industry. Krita has been in development for over 10 years and has had an explosion in growth recently.