Your actual question is a bit confusing, but parts of it are answerable.
If your parents give you cash to buy a home, you use it for a cash sale, and you then repay them, then if they don't charge you interest (above and beyond any they may pay - treat their loan as entirely separate from yours.) that amount of not-charged interest is considered a gift. You can look up these rates here, on the IRS website, called "Applicable Federal Rates"; currently it's around 2.30% APR. If the interest is that much, the interest isn't a gift, but part of a loan - though your parents will have to file some paperwork and you will also. Search for more information about "Intra-Family Loans" for full details - and talk to a real estate lawyer.
Separately, if your parents are the primary/only signors on the mortgage, and are thus effectively the property owner, there are two separate issues. First, you're welcome to pay off the mortgage while you live there, and no tax issues exist (then) in terms of gift, though it's still owned in your parents' name - so you don't get the house, they'd have to gift that to you separately, though they can do that in stages to avoid gift tax ($14k worth of equity per year, or whatever the current year's annual limit is, per parent and per you/wife if applicable).
H&R Block discusses the concept of Equitable Ownership, which determines who is able to take the property tax and mortgage interest deductions on the house. Tax Almanac goes into a bit more detail on the subject. Ultimately, another issue for a real estate attorney probably (or at least a CPA or tax attorney): you need to be very careful if you're going to try and get the tax deductions (which would be probably far more than the savings of a half percent of interest rate or whatnot).
In general, there are a lot of options for exactly how you structure the purchase, payments, and equity, and you should talk to a professional to find out exactly the right way to structure it. This mortgage info site has some good tips for several different ways to structure things, just as an example. I would in particular suggest you pay attention to deductability of mortgage interest and property taxes, as those two can be a huge amount of tax savings and you can lose that very easily if you're not careful.