The term gantry, referring to a gantry crane, a bridge crane supported by two or four movable legs, is derived ultimately from the Greek word kanthēlios, meaning a load-bearing animal, specifically a "pack ass". Early rockets were assembled on the launch pad by such a movable crane, then as time went on more work platforms were added until it became the mobile service tower of today. SpaceX however does all servicing in the horizontal position and has no mobile service tower.
Comple 17B (I believe) accommdated both the Delta 2 and Delta 3 (Sorry, I meant the Delta II and Delta III) event hough the latter had a much larger upper stage. Cx17 was somewhat unique in providing servicing for loading almost any propellant; LOX, RP-7, LH2, N2O4, and Aerozine-50. Comple 37 accommodates both the conventional and heavy versions of the Delta IV, even thought he latter has three boosters and the former only one. In both cases this was accomplished by movable or fold-up work platforms that could be used in various positions.