I have a 1990 coleman tub. I am Rebuilding it. It is a 120v that i had pluged into an outlet thats non gfi. The outlet and plug have 1 sideways prong. My old spa pack had a gfi right on it. Now this new pack i am installing has no GFI. So my question is what should i do. The spa is on an enclosed deck. Can i just get a 15 amp gfi breaker and put that in my breaker panel ?

The plug and recepticle you describe is the old NEMA 5-20, which is rated 20 amps. Most of the newer plugs and outlets in homes are NEMA 1-15, or 15 amp ratings.

You could put a new 20 amp GFCI in your service panel (if you are using 12 Gauge wire), but they are pretty expensive. I would just install a 20 amp GFCI in the receptacle on the deck, and that way you could test it once a month (by pushing the TEST button). These are much less expensive than the ones that mount in a service panel.

The 20 amp GFCI is much more robust than the regular 15 amp models that electricans install in new homes, bathrooms, decks, garages, etc. It will accept the sideways prong.