It won't be a giga. SeaWorld won't have the money to spend.
If they believe (and can show with calculations) that it will increase visitor numbers and/or justify a gate price hike, thus increasing their income (and they have the calculations to back it up with), it wouldn't be too hard for them to get a loan. A Giga coaster is an expensive construction, indeed, but less so than, say, a mid-size office building or a large-ish hotel. Property developers take up loans for investments all the time.