Pokemon Grey is just fan speculation.
The "reasoning" behind Pokemon Grey is that all previous generations have had trio games:
i. Red, Blue, Yellow
ii. Gold, Silver, Crystal
iii. Ruby, Saphire, Emerald
iv. Diamond, Pearl, Platinum
However, with the announcement of Pokemon X & Y, it's unlikely that Nintendo will go back to Gen V and add an extra game.
Also, there has been Pokemon Black & White and Pokemon Black 2 & White 2. There are already 4 games in Gen V, and so it seems to be over populated with main series games in any case.
Pokemon Grey has no official bearing; Nintendo might release it, but I find it highly unlikely.
Pokemon Grey - Official Nintendo Magazine website
^ this article was written in 21st Feb 2012, and B2W2 were revealed on 26th Feb 2012. Speculation about Pokemon Grey came before the release of B2W2.
Also, Reshiram and Zekrom are the opposing factions of the original Dragon Pokemon. The only reason one of them can fuse with Kyurem is because Kyurem has a void that needs to be filled by one of the Dragons, not both, since the reason why the original Dragon Pokemon split was because it could not hold onto both truth and ideals.