User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-28303251-20150620032351/@comment-5569775-20150621160752

I unlocked it as soon as I got to G1, which was also when I first bother checking with the Gong Lady.

A friend IRL was talking about the guild card yesterday and how there's a reward for doing so many different quests, that changes the background to white and another one that changes it again later, also needing a certain number of different quests to be done.

So it's possible The Gong is also upgraded based on the number of different quests done. Though doing a good amount of quests of a particular rank also might help.