Well i just figured something out also.. Normally you touch the ground cuz you want to have your summon land "softly""..

Therefore its not needed to touch the ground or whatsoever to summon. You can stand on a cliff do the summon jutsu in mid-air and poof there goes your summon down the cliff

Well since naruto had no choice and gamabunta is big enough they got saved

.. you get the point, its just a matter of safety and comfort for your summon to do touch the ground