The export works, and it is unusually complete for a thing a company gives you on the way out. The difficulty is entirely at the other end: what arrives is a format built for mail servers in the 1980s, and the tools that read it comfortably are mail clients you probably do not want to install to answer one question.
Asking for it
Takeout covers every Google product at once, so the first step is to turn almost all of it off. Deselect everything, then select Mail alone. Mail then offers a choice between all of it and specific labels, and the label list is the way to get one project or one year rather than fifteen years.
Then three settings that matter more than they look:
Delivery. A download link by email, or straight into Drive, Dropbox, OneDrive or Box. The emailed link expires after about a week and allows a limited number of downloads, which is a real problem for a file of this size on a slow connection.
File type and size. Zip or tgz, and a maximum size per archive file from 1 GB to 50 GB. Anything larger is split into several archive files. That setting is about how the download is packaged, not about how your mail is grouped — it does not divide the messages up in any way you would find meaningful.
Frequency. Once, or every two months for a year. The second is the better answer if you are leaving and have not finished leaving.
The build takes anywhere from minutes to days depending on how much mail you have, and the result is a snapshot: mail that arrives after you press the button is not in it.
What is in the file
Inside the archive, Takeout/Mail/, and inside that one file, usually All mail Including Spam and Trash.mbox.
An mbox is the simplest possible archive: every message written out in full, one after another, in a single plain-text file. Each begins with a line starting From — that word and a space, not the From: header — and the reader treats that line as the start of a new message.
The format predates Gmail by decades and belongs to nobody, which is the good news: it is genuinely portable, and Thunderbird, mutt and a dozen other things have read it forever. The bad news is that it was never given a strict specification, so variants disagree on small points, most famously how to escape a line in the body of a message that also happens to begin with the word From.
Labels are headers, not folders
Gmail has no folders and mbox has no folders, so the labels are recorded the only way the format allows: as a header on each message.
X-Gmail-Labels: Inbox,Receipts,Important — a comma-separated list, mixing the labels you made with the system ones. There is usually a second Gmail header, X-GM-THRID, holding the thread identifier that grouped a conversation together in the web interface.
Two things follow from this. The first is that anything reading the archive has to read that header to know where a message lived, and most mail clients ignore it on import — which is why importing an export into a new client so often produces one enormous undifferentiated pile.
The second is more welcome. A message with three labels appears once, with all three in its header, not once per label. The message count in the file is your real message count, and there are no duplicates to clean up afterwards.
What is not in it
Contacts. A separate product in the same export tool, selected separately, delivered as vCard or CSV. Exporting Mail alone gives you addresses only in so far as they appear in messages.
Drive files. Also separate. This matters more than it sounds, because a large attachment sent through Drive was never an attachment — the message contains a link. Export the mail and you have preserved the link to a file you may later remove.
Most settings. Filters can be exported, but from Gmail's own settings screen rather than as part of the archive. Signatures, the vacation responder, forwarding addresses, POP and IMAP configuration and label colours are not exported at all. If the account is going to be closed, those are worth writing down by hand, because nothing else is going to record them.
The size problem
Messages are small. Attachments are not, and they arrive inflated: mail cannot carry binary data, so every attachment travels rewritten as text, about a third larger than the file it came from. A decade of ordinary correspondence with photographs in it lands somewhere between 5 and 30 GB.
One file of that size is awkward in every direction. A text editor will try to load the whole thing into memory and stop responding. Search tools skip it or index it as a single document. And you almost never want all of it — you want the invoices, or the messages from one person, or the eleven you need for a dispute.
Which is why splitting it into one file per message is usually the first useful thing to do with it. It is not a conversion: an mbox already is a stack of complete messages laid end to end, so splitting takes them apart rather than transforming anything. What comes out is a folder of .eml files that your operating system can list, sort, search and open one at a time.
Reading it without installing anything
Three approaches, depending on the question.
To look through it, a viewer that streams the file lists the messages and opens them individually, without needing the whole archive in memory at once.
To count and sort, turn it into a spreadsheet of sender, recipient, date, subject and size. This is the fastest way to find out what is actually in fifteen years of mail: sort by size to find the attachments worth keeping, or by sender to see what can go.
To keep a few messages, split the archive and then work on the individual files — reading them in a viewer, or turning one into a PDF with its headers on the page if it needs to be filed somewhere as a record.
The one thing worth avoiding is uploading it. This is not a document, it is your entire correspondence — every password reset, every medical appointment, every private conversation you have had by mail — and putting several gigabytes of it on a stranger's server to read one message is a poor trade, quite apart from how long the upload takes.
The short version
Takeout gives you a real, complete, portable archive in a format from before the web, with labels demoted to headers and everything except mail left out. Nothing on a normal computer opens it. Split it, list it, or convert it to a spreadsheet — and do that locally, because the file is not a document, it is all of them.