So far, quite a few people have said it might be 47.
No.
The game's mascots are the only.. How to put it.. Event-triggered Pokemon (as in you speak to them to initiate the battle) who are at level 47.
This is seen in Ruby, Sapphire, Diamond, Pearl and Platinum. (FRLG have no signature legendaries, and Emerald kept Rayquaza at level 70). Gyarados is not a legendary, it's level will most likely remain the same, although perhaps pushed up to a higher level of 35 or 40. How many wild Pokemon level 47 do you meet before even the 7th Gym?? (Actually, there's Articuno and Zapdos in FRLG, although neither of them are compulsory battles). Exactly, you don't. So although levels of everything will be boosted (probably), Gyarados' level will not change drastically.
Well i don't need to say now that i think Gyarados'll be included. The Red/Shining Gyarados looked awesome in DP (my friend cheats) and hopefully it will look similar.