Grànnda according to AFB, Dwelly and Mark. No mention of grànda anywhere. Where there is variation, Mark sometimes slips a variant into an example without noticing, but even this hasn't happened. It is safe to say (and this rarely happens) that there is no choice so whatever. It looks like a typo in learngaelic. Is there a procedure for querying their spelling on their website?
Edit: having done an internet search for grànda bha, it does occur in quite a few places, including a recent translation of the Bible. But that does make it right.
(Note that bha is an extremely common word in Gaelic, but it does not exist in any other language on Earth - not even Irish. So including it in a search string is a quick and reliable way of restricting search results to Gaelic.)