Things have changed a bit since I wrote what I did when the thread started, but not much.
It's still easy to get Gas Safe Registered.
People regularly either do some noddy "gas" course, and/or lie their way with an accomplice who's GS registered, and read a book.
If you're fairly bright - capable of doing one of today's "A" levels, say, you can learn all you need in a day with a book, and a couple of hours with someone showing you the right things.
It's supposed to need longer than that, but it depends what you know and how quick you are to learn. Some people need years.
Then you go to an ACS centre as a Category 2 person ("Working in the industry") and get some "training" where the trainer knows what you're going to be assessed on, so he tells you what you need to know.
It's immediately obvious to an assessor how much experience you have, but that doesn't matter, you just have to get the answers right.
This is not conjecture on my part, it's precisely how some people get qualified. I know because I'm an assessor and I ask them once I've passed them.