A GdkRGBA expresses the color and alpha channels as fractional [0, 1] intervals, instead of integer [0, 65535] ones; to convert from GdkColor to GdkRGBA, you can use this simple snippet:
You’re strongly encouraged to migrate all your code related to colors to GdkRGBA, but of course API taking a GdkColor will keep existing in GTK 3.x. It’s only going to be an issue if you’re going to port your code to GTK 4.