This game appears to be gone from one of the few places where it can be bought!
I agree it is difficult to understand why this situation even exist.
It is known that BIS and Codies both own some part of Operation Flashpoint, and they are not friendly companies to each other anymore.
I would have understood if this situation happened if gog was an abandonware websites, as not every company is altruist enough to accept their product to be released basically for free even if it has not been supported since 2005, most of them are in this business for money primarly and OFP is always a potential good seller.
But in the case of such commercial websites like gog that is an online store selling games, both BIS and Codies would continue to earn money from the sales like from any stores selling a software.
Unless there is something else that gog does not tell, and the problem may not be on BIS/Codies side.