Hi Rebecca,
There is another technique that you can use that could help to display the information you are looking for. Report link fields allow you to display a report pulled from another table by looking for matching values in a field in one table to a field in another. In this instance your Contacts tables email address and your gmail sync tables email address field could be used so that when you are viewing the Contact record you can see a report of all the records in the gmail table with the matching email address (This can even work without a relationship). The downside of this method is that it doesn't build a relationship so you aren't able to make use of functions like summary and look up fields but it does at least help gain some visibility for your users.
You would do this by creating a new field on the Contacts table with the field type Report Link. Then when you open up that new Report Link field's settings you are asked to pick which field in the Contacts table you want to match to which record in the Sync table. Then it will use those two matches to build a report to show all the email records for a contact on the contacts record. It will show the defaults to start which might not be the exact fields you are looking for so you might have to follow this step up by tinkering with the report till it shows exactly what you are looking for in regards to fields. Long term you may still decide you need to be able to have contacts and emails linked but in the short term it can help increase the visibility of those records in your app. I hope this suggestion is helpful.
