I had an extraction with bone graft 6 days ago, can I smoke if I put a gauze over the affected site?

I really doubt putting gause or anything else over the site will make any diference. 
If you have had implants  placed , ideally you should give up smoking and if this is not possible try to avoid smokling for a couple of weeks following placement  especially if a bone or soft tissue graft  procedure has been performed. 
Smoking will affect the healing process and may result in a poorer final result. Long term  smoking especially if yoa rae  a heavy smoker  increases the chances of implant complications. Some dentist who place implants will decline to do so on smokers for this reason.

Smoking can cause the bone graft to fail. If that happens, the graft material would need to be cleaned out and replaced, which would most likely be an additional cost. I advise my patients not to smoke for at least 2 weeks after any oral surgery. Placing gauze over the site may prevent any issues due to the sucking motion of smoking, but the actual chemicals delay healing significantly and that's the real issue 6 days out.