There is little that prevents a spammer from sending an email with a forged senders' "From" address. (Well, actually, it's a little harder than it was 10 years ago, but it is still possible.)
The cretin / spambot that is using your e-mail address could have harvested it from any place that it was made publicly available... or perhaps harvested from the email store on a virus-infected PC owned by someone who has received an email with your address listed somewhere.
You would probably need to be using a gmail address that had never been used even once - for any purpose - before you conclude that "gmail on my phone was hacked".
I'm not saying it is impossible, but that there are a lot of other ways it could have happened that are more likely than the route you are suggesting.
(Note that at least one me those ways could be a rogue app you installed on your phone that has access to your address book)
eu1