1) Yes, if the branch circuit conductors are all 12 AWG copper, a 20 amp double pole GFCI can be installed. However, the load amp rating determines the maximum size of an over current protection device. If the connected loads state that the maximum amp rating is 15 amps, then only a 15 amp breaker can be used.
2) 15 amp double pole GFCI breakers are available but are limited to only a few panel manufacturers. Thus the reason you are not able to locate them at Depot or Lowe's. I recommend to check with a a local electrical supply store as they have better inventory than the big box stores.
Siemens makes a 15 amp double pole GFCI breaker, but it may not be allowed or fit in your panel. 20 amp double pole GFCI's are more common than 15 amp double pole GFCI breakers. Breakers installed in a panel must be the same manufacturer as the panel manufacturer. Foreign breakers installed in a panel are not listed. In other words, you can't install a Siemens breaker in a GE panel even if it fits....ie... a code violation.
3) Worst case scenerio, install a 15 amp double pole breaker using (2) two separate 15 amp GFCI receptacles and wire the pool light and spa lights to the GFCI LINE side and they will be GFCI protected. Refer back to my original diagram.
If you have any additional questions, just let me know and I’ll be glad to answer them for you.
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