I had FUE of 2700 grafts 15 days back and now while head washing I find hairs on my gauze. Is it normal?

You need to see your doctor for at least a recheck.Why would you still have gauze on your head (after 15 days).In general it is normal to see some hairs (not grafts) falling out after surgery. It is not normal to see many grafts falling out after surgery (2 or 3 is OK).

you are right on track.  in fact, it is best for these hairs to come out.  If they don't come out, they will lead to pimples and cysts eventually because non-growing hair is a foreign body.  Get rid of these hair follicles.  After 6 days the grafts are in solid and you can't pull them out.  Use a gauze, wash cloth or even tweezers to get rid of any hair follicle that is not growing.  You will know it is growing only if it continues to elongate.  If it just sits there, then it is not growing and you should do your best to get the follicles out.  The follicles will have a characteristic "J" shape at the end.  These are dead, non-growing hairs.  

Don't worry about your transplanted hairs falling out at this stage as it is quite normal. They should start to regrow in 3-4 months so you will need to be patient.