gpu.bindTexture(textureID) -- Bind the GPU to texture (textureID)
gpu.createTexture(width, height) -- Create a texture of size width, height
gpu.drawTexture(textureID, x, y, [sx, sy, w, h]) -- Draw texture (textureID) at (x, y) starting optionally at (sx, sy) and length of w, h
gpu.fill(r, g, b) -- Fills monitor with specified color
gpu.filledRectangle(r, g, b, x, y, w, h) -- Makes a filled rectangle at x, y with dimensions w, h
gpu.flipTextureV() --
gpu.freeTexture(textureID) -- Delete texture (textureID), range 1 to 8191
gpu.getBindedTexture() -- Get the currently binded texture. Returns textureID
gpu.getBPP() -- Get the GPU's current BPP setting. Returns BPP
gpu.getColorRGB(x, y) -- Get pixel color at (x, y). Returns r, g, b
gpu.getFreeMemory() -- Get the GPU's free ram status. Returns remaining bytes
gpu.getNativePixel() --
gpu.getPixel(x, y) -- Alias for getColorRGB
gpu.getSize() -- alias for getSize()
gpu.getTextureSize() -- Get size of the currently binded texture. Returns width, height.
gpu.getTotalMemory() -- Get total size of ram. Returns remaining bytes
gpu.getUsedMemory() -- Get the GPU's used ram status. Returns used bytes
gpu.line(r, g, b, x1, y1, x2, y2) -- Makes a line from (x1, y1) to (x2, y2)
gpu.plot(r, g, b, x, y) -- Alias for setColorRGB
gpu.rectangle(r, g, b, x, y, w, h) -- Makes a rectangle at x, y with dimensions w, h
gpu.setBPP(BPP) -- Set the GPU's BPP setting.
gpu.setColorRGB(r, g, b, x, y) -- Set pixel at (x, y) to r, g, b
gpu.setPixels() --
gpu.setPixelsYX() --
gpu.setTransparencyColor(textureID, r, g, b) --
gpu.setTransparent(textureID, transparent) --

Notes:
BPP is "Bytes Per Pixel" and not "Bits Per Pixel"
TextureID 0 is the texture of the main monitor. Bind to this to draw normally.