So you sold your GW, to buy a Sky, and now you're planning to buy a GW down the line?
Someone's a little impatient with the GW team xD
At this moment in time, you can't go online without updating your game, and you won't be able to do this since you're not on 9.4.
Sky3DS has no way of allowing you to download updates without being on the latest firmware at this point.
Guess you'll just have to wait until GW team releases NN3DS support (and also wait for GW flashcart shipping
EDIT: To answer your additional question, the Sky disk writer tool allows you to backup your saves. I may be wrong here but I'm sure you can use those save files with gateway by pasting it (named correctly) in the root of your SD card.