You can plug you SD card in your PC and check its partitions. DON'T FORMAT ANYTHING, EVEN IF WINDOWS TELL YOU TO DO SO (not sure it would, but let's not take the risk). IF you either have two partitions, or one plus "unused space" well your emuNAND is either one of these partitions or that "unused space".
If you don't have an emuNAND anymore, well if you really want to use your Gateway again (see above posts for the few reasons why you'd want that), you need to create an emuNAND again. Which you can do from Gateway's menu, as usual (there are community tools too such as GodMode9, Emunand9, Decrypt9, Hourglass9, etc, but if you know how to use that, just use that). If you don't and think Luma is the way to go, don't bother creating an emuNAND and live your life on sysNAND.