Hi,
Fixing the DNS and reverse DNS records is the straight-forward part here (I have worked with them before with my own clients). Depending on if the issue is with Gmail, it may not be as simple as just fixing these, though.
I recommend these listed issues to be fixed first though, and then see if the issue has been resolved - especially the SPF part can give serious issues on the receiving end if not configured correctly (=dropped emails/flagged as junk). If not, we can look for other issues - they are usually simple things, but are not always easy to find.
If you can mention which domains are affected, I can do some DNS lookups and give an estimate on how long it will take to fix it.