Looking for a late 80s/Early 90s fantasy film:

 - Boy is given necklace with green gem from grandfather
 - necklace opens portal to fantasy world in basement
 - Boy befriends frog creature
 - Large Zeus-like giant king on throne

Anyone have any ideas? I have already gone through all of the "80s/90s Fantasy/sci-fi" lists on IMDB, and have searched every combination of search terms on Google.