The GX24D is a 2-pin cap with a 24mm space between pin centres. It comes in three different forms: GX24D-1, GX24D-2, and GX24D-3. Each form has identical pins, but has an outer plastic guide that will only correspond with the appropriate lamp holder, so care must be taken to buy the correct type.
GX24D-1, GX24D-2, GX24D-3.
2-Pin Triple Tube CFL (Triple Biax).