@Nicholas_Doss, @TomaszGolos and @glipka: The bug you all reported happens when the ETag (https://en.wikipedia.org/wiki/HTTP_ETag), which is a header in the http request, changes between the moment the activity has uploaded the email as a draft and the moment it actually sends the draft. This can happen for a variety of reasons, the most common of which is actually modifying the draft between the time it is uploaded and the time it is sent. However, you’d have to do some pretty whacky workflows to actually do this.
There is also this albeit older issue on the google client SDK GitHub in which other people have gotten the exact same error using other services (Calendar in that instance) and the problem resolved itself.
So I think Dean Mauro is right in his statement that this is a server side problem that will remedy itself.
I have just tested this by running the Send Mail activity in a loop 100 times, and I have gotten no error, so I think the case is even stronger for a server side issue.
Please let us know if this continues, as currently we are not investigating this as a bug on our side any longer!