What exactly do you need to role to climb on top of and stay on the back of a Gargantuan beast when you have Slippers of Spider Climbing on your Medium-sized character?
The target creature does not want you on his back...and presumably you will be attacking the creature.
And at my table you would get a bonus under The Rule of Awesome ;-)
But if you have a climb speed then why would you need to make Climb checks? Obviously if the creature Bull Rushed you or Repositioned you you could go flying.
But if you have a climb speed then why would you need to make Climb checks?
Because having a climb speed does not remove the need to make climb checks. It just makes them easier.
A creature with a climb speed has a +8 racial bonus on all Climb checks. The creature must make a Climb check to climb any wall or slope with a DC higher than 0, but it can always choose to take 10, even if rushed or threatened while climbing. If a creature with a climb speed chooses an accelerated climb (see above), it moves at double its climb speed (or at its land speed, whichever is slower) and makes a single Climb check at a –5 penalty. Such a creature retains its Dexterity bonus to Armor Class (if any) while climbing, and opponents get no special bonus to their attacks against it. It cannot, however, use the run action while climbing.
How about just step on a vial of Sovergn Glue...and call it a day :-P
Sadly there is no real hard and clean rules for climbing on the back of really large monsters. Grapple is close, but has size limitations and imobilizes the gargantuan creature, not what you hope for. Ride checks are also kinda close, but is more geared towards more willing mounts and does not take into consideration the thing attempting to buck you off, attack you or be attacked by you. climbing is also ill suited to this, as it assumes you are climbing an inanimate object or something like a cliff face. Again, it does not take into consideration the thing attempting to buck you off, attack you or be attacked by you. Since none of these really address the issue and the developers seem ok with that, it is left to the realm of GM fiat or houseruling. A good one that i have seen is a cling manoeuvre, like grappling, but your target is not grappled, just you. i will leave you to decide any modifiers or rules for that, as this is the rules forum, not the houserules forum.
Btw, the idea to use spider climb is really creative, and as such I would reward it, but again, no rules really support it :P pity.
I would say using Climb and giving it a hefty penalty, because it's like trying to climb something during an earthquake fit's best.
It's not perfect, but as Gobo Horde said no solution really is since there's no rule covering this situation.