The Bow River is arguably the best trout river on the continent, however I do not believe there are any grayling there.
Exzcellent time of the year to go actually, before the runoff begins and the river should be in good shape.
5 or 6 wt. would be best but a decent 4 wt. should be OK.
There are a number of local fly shops and guides that would be able to help with fly selection.
You shouldn't be disappointed with fishing the Bow, browns & rainbows, and lots of them, up to 5 - 6 lbs.
Best of luck.
Paul