How I archived my Gmail account before I left it.
First read this:
https://support.google.com/accounts/answer/3024190
Then chose the "Mbox format" for email. Then grab a copy of Thunderbird Mail app. I use this for archive purposes only. Then use the built in import function in Thunderbird. Why? because Thunderbird is very good at importing a huge amount of data, and indexing it for later search.
