Is there a way to track download of our mobile apps in iTunes and Google – based on an email campaign? We would be appending a GA tracking code to the URL in the email.
Not in Marketo. You could make a causal link, but not an ironclad direct one. If you looked at the list of people you emailed and then compared it to the list of people who then installed the app (using the Installed App trigger), you could reasonably conclude one impacted the other if it was in a short time period, but I can't think of a way to directly close the loop.
I'll have to leave it to the GA experts to tell you if GA could help you here.