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6/6, prevent all damage dealt to him. No other abilities.

Gideon Jura's +0 modifies the soon to be animated Gideon's P/T and abilities at a later time stamp than Humility; and so they supercede.

Humility
Enchantment, 2WW
All creatures lose all abilities and are 1/1.

Creatures are only creatures on the battlefield. In all other zones, they are creature cards. Note the distinction.

No, Rock Jockey only loses its abilities once it's on the battlefield. It still has a casting restriction.

Death triggers (When ~ dies) are also wiped by Humility, since they are originated from the battlefield.

It would not gain any new abilities. Humility still takes effect prior to the layer that grant those abilities. It would retain its new P/T settings until EOT however.