gp with sated devourer?

he can already 1 shot with q, BUT makes his q crit/magic dmg/ludens proc.rylais.. also his heal scales 100% off ap so 1 k heals.

It doesn't work with GP and ezreal Q because they are not AAs. They are abilities that proc on hit effects. This is different from other abilities like Jax W which is an AA enhancer. Also, Jax W doesn't proc twice either. The "double on hit" effect is actually just adding another auto attack but with 0% AD scaling instead of 100%. This means that on hit effects that gets consumed only proc once, but you will for example get an extra stack for things that stack with auto attacks.

It doesn't affect Gangplank's Parrrley.

Sated Devourer will see the most usefulness for champions like who all build on-hit effects or have in-built mechanics in their kits which allow the usage of the item to its maximum potential.

Or if you wanted to join the meme meta, build it on and napkin theorycraft Silver Bolts procs every 2 attacks instead of three.