Yes Gimp is free and I use it to make textures, there's some nice packs and themes that you can install to make it easier to use and with new features like the ink pen and blending in Gimp.
I've also tried the new versions of 4th paint and nekopaint and both have way better selection tools than SAI, you can select in non-contiguous mode, meaning that if, lets say your character has the same blue color for the shoes and eyes, you can magic wand the shoes and both the eyes and shoes get selected, this is specially useful if you work in cel-shading and need to select various parts of the same color to edit (change color for example), in SAI you have to select every small bit if you want to pick all the color.
SAI has been too quiet, it could have at least gotten a couple updates like better selection tools and remove the 100px binary pen limit, but it's almost an year without any update.