You have to have caught one for it to show up in the DexNav, secondly it will only show up in hidden encounters (where a little sprite appears in the world and you have to sneak over to it) you can't just encounter it normally.
Also, if you're trying to tap on the Gible in the DexNav it may not work until you've captured or KO'd a few more, the search number usually has to be around 10 to get them to appear and around 20 for them to appear reliably.