Before I start, I have to point to the disclosure below my signature: I cannot give advice, and any decisions you make based on the information I provide are your own, not my recommendations.
That said, it sounds like you're getting $891 in savings (not sure how you arrived at the $33/credit figure) with 27 credits through the GSP plan, plus you will be able to claim the Pennsylvania tax deduction for contributions to the plan as a resident. Contributions up to the gift-tax annual exclusion amount ($14,000 in 2017) per beneficiary are deductible in computing Pennsylvania taxable income. Spouses filing jointly must each have at least $14,000 in income to claim the maximum $28,000 per-beneficiary deduction. Note that the deduction is available no matter which 529 plan you might use in any state.
However, you are giving up some versatility when you open the GSP plan. The assets are no longer liquid, and will be subject to tax on earnings plus a penalty if you don't end up using the money for qualified expenses for most reasons (exclusions to the penalty apply in the event of death or disability). So if your daughter switches schools - which would change the math on the benefit, I believe - falls in love and elopes to Europe, decides to live off the earth, becomes a starving artist, etc., you'd have some decisions to make about the account. There are also some fees associated with the plan, including a $25/50 fee depending on how you open the account, and additional annual fees taken out of you credits (details here). Not sure if you accounted for these fees in the math.
So you have to decide if the additional administration and reduced liquidity of the assets are worth the $891 in savings plus the value of the state tax benefit (not sure what your tax bracket and planned deposit amount are, or I could quantify that for you, but it's probably between $200 and $300 in state tax savings).
Brian Boswell VP, Research & Development
This information does not constitute tax advice and is provided for informational purposes only. Please consult your tax advisor, financial advisor, local taxing authority, and/or plan provider or sponsor for more information.