Agama materials is described as 'is software for creating PBR materials and painting 3d models. You can create materials and use them for painting models, or you can use pre-made textures. Simple creating of procedural materials'. There are four alternatives to Agama materials for Linux, Mac and Windows. The best alternative is Armor Paint. It's not free, so if you're looking for a free alternative, you could try Quixel Mixer. Other great apps like Agama materials are Substance Painter (Paid) and Union Bytes Painter (Paid).
Union Bytes Painter allows the creation of 2D sprite and low res textures with color, normal, height, roughness, and metallic maps. Sprites and textures can be edited using multi-channel mode in the 2D or 3D space.