Tax deductibility for a donor is based on giving to a nonprofit; it's not dependent upon how that nonprofit uses the funds. On GiveGab, the nonprofit pays for the fees, regardless if the donor elects to cover. GiveGab gives donors the option to increase their donation by an amount equal to the fees the nonprofit will pay so that the nonprofit realizes the full amount of the intended donation after payment of the fees. Just because the donor is given this option does not make the additional amount non-deductible.
Outside of using GiveGab, nonprofits do not provide their donors a breakdown of what portion of their donation is deductible (unless the donor receives something of value in return as a dinner, etc). That's because deductibility is based on giving to the nonprofit, not how the nonprofit chooses to use the funds. You could almost consider when the donor elects to cover the fees, that portion of the donation is a restricted gift that will be used to pay for the expenses incurred to receive the gift. Restricted gifts are tax deductible, because it's based on who is receiving the gift (nonprofit) not what the gift is used for.
Ultimately, whether a donor decides to deduct the fees as a donation on their tax returns is their option, and we do not provide tax advice. If they want to be ultra-conservative they can choose to not take a deduction for the fees when they file. Who receives the donation is what determines tax deductibility, not how the donation is used. If it were the opposite, nonprofits would have to provide a full accounting to each and every donor how every penny of each donation was used - that is obviously impractical and would increase nonprofit administrative expenses exponentially.
GiveGab does not provide tax advice. Please consult your local tax professional about what portion of the full donation amount can be claimed as a charitable contribution.