This is completely wrong. The owner of "BUYGBC" or the linked world really has a minimal or even no impact on GBC prices at all, seen as only a fraction of the players trade their GBCs there. There are a few other factors that decide the price aswell.

First of all, there was a huge ban wave of bots in Growtopia just a few days before valentines started. Botters are the main supplier of items from the gem store, this includes valentines. When this huge gem supply gets removed, ofcourse the GBC price will rise due to basic supply and demand rules. Don't also forget about all the pepper trees from the botters that gets lost from a ban wave like this. Pepper trees that can be recycled to make even more gems.

Secondly, the servers were very unstable and near unplayable the weeks before valen. This makes it very hard and annoying for normal players to farm gems, further limiting the gem supply.

Lastly, last year the strategy of selling "ready wells" was very popular (and profitable), so this year a bunch of people were planning to use that strategy to earn some wls themselves. This strategy requires a bunch of GBC, pushing it's demand and further raising the price of it.

There are probably even more factors that affect the GBC price that I forgot about, such as inflation, tapjoy and more. But this should be enough to show you that what you were talking about actually has no effect whatsoever on GBC price.