There are a handful of programs and purposes-- the national debt, some National Parks stuff-- where Congress has authorised the Treasury to accept donations and then spend the money for that specifically earmarked project. Otherwise, gifts to the government go into the general fund just like tax payments.
There is no legal mechanism for one to donate to the border wall in particular, and Congress obviously isn't going to create one any time soon. It's kind of amazing GoFundMe hasn't yet shut down that fundraiser and returned all of the money, because there's simply no possibility for it to work the way it's being advertised. A GoFundMe page doesn't let you bypass the constitutional prerogatives that are vested solely in Congress.
"No Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law..."