This is a backport (adapted to GTK+2) of the fix brought in GIMP 2.99.8 for Wayland and macOS. Showed on various screens or even printed, subpixel font rendering doesn't make sense. Yet when rendering an image which can be zoomed in or out, These systems are useful for GUI rendering on a screen of a specific type and pixel order.
Do not follow subpixel font rendering choice from system settings for text layer rendering.