The short answer is don't hold your breath. If you really want a P99 style gun in .45 ACP, you should look into getting the SW99 (standard DA/SA type of action) or SW99OL (QA type of action).
Regarding the .45 GAP cartridge, I think that's a wait and see thing for many companies. Some companies are getting into it and others are waiting to see if the market will accept the cartridge before adapting their 9/40 pistol platforms to the GAP. It seems every few years someone comes out with the next, new wonder cartridge. The manufacturers don’t want to “tool up” for a gun chambered in a new round until they know the cartridge will be embraced by shooters. If you look back at some of the newer pistol cartridges, the level of acceptance by shooters is spotty. While .40 S&W is now commonly encountered, .41 AE is essentially gone, 10mm is basically a niche market, and although .357 SIG is being adopted by some LE agencies due to price incentives, it hasn't yet become a common caliber in the U.S.
Since that’s the situation, I wouldn't expect to see that much offered in .45 GAP until it becomes better established in the marketplace. As for the .45 ACP, Walther has been continually asked this question by Americans and, to date, they have not produced one. It does not mean they never will, but I do not see it as a high priority for them.