Do you have the mm cart? If transferring a gateway save to a cart is the opposite of transferring a cart save to a gateway then:
1) locate the .sav on the consoles sd card, move all .sav's to a computer and move one at a time back, every time launching Mm to see if the save is back.
2) when the save reappears in the rom, that means you transfered the correct .sav to the sd card, copy that to a new location on the computer, and transfer the other gateway saves back onto the console SD card.
3) keep a backup of that save in 2 places in case something goes wrong or it doesn't work.
4) on the label of the game cart, on the bottom there should be a number like: XXX-XX-####-USA
Those #'s come right before the game region, like on mario kart it's XXX-XXX-AMKE-USA, we don't need the numbers which I labeled as X's or the region. We want the 3-4 letters right before the region (like the AMKE from the mario kart example).
5) once that's located you will want to rename the gateway MM .sav file to ####.sav (like if it was mario kart it would be AMKE.sav) and put the file on the console SD card.
6) now power cycle your 3ds so it's in sysnand.
7) launch the gateway menu NOT GATEWAY MODE.
8) put the MM retail cart in the system and find restore cartridge .sav or similar (might be restore cart save game, you get the idea)
9) click the option, it will find the ####.sav (y'know whatever you actually named it) and load it on the cart.
If this doesn't work, or something goes wrong, you have a backup gateway save, so you can always still play it one your gateway. And you can't mess up a cartridge by editing the save, worst case scenario, you start the game it says the save is corrupt, it formats, it's done.
Oh and if you had a save on the cart, you can back that up in case something goes wrong and you want the old save back on the cart. All you do is go to the gateway menu and hit backup cart save game. But don't do this when the save you want to restore is on the sd card, or it will be erased.
