One thing you can do to track the conversions for gmail is the create a separate adgroup for gmail and use the placement targeting feature to only target gmail.
If your other adgroups are using the relevant placements across the network or you have selected gmail in these older groups I suggest you exclude gmail from these groups by going to your TOOLS and using the site exclusion tool. If you do not see a TOOLS menu on the interface look for OPPORTUNITIES and then see the left side of the page for your TOOLS.
What I am getting at is only use one group to specifically target the gmail domain and make sure all other groups are not targeting gmail.