Is there a tool capable of drawing a triangular linear gradient fill? I need to draw some shapes filled with linear gradients starting from every point and blending smoothly between points, like an OpenGL standard shading (I guess it is called GL_LINEAR or VERTEX_COLOR), i.e. GIMP / Illustrator / Krita / Paint3D, etc. Something fast and simple.
required result:
Most 2D graphics programs are able to do linear gradients with arbitrary orientations. If you don't mind a little work, it is possible to set this up to imitate the 2D linear interpolation across a triangle.
Set up two layers:

Pick two of the colors, say red and green, and set up a linear gradient between those two colors along the red-green edge.
In a second layer, set up a linear gradient between blue and transparent-blue (i.e. the same color with alpha = 0). Orient it perpendicular to the red-green edge.