videos say to flutter lips and say "brit" or "briiiiiiit".

this ends the purr on a high note. I found it relatively easy to master. I cannot do the gargling thing...

Use pressure of tongue to change pitch. I like a little higher pitch that actually drops in tone at the end. This is the sound I have heard turkeys make in the wild more than ending on a high note. So I usually just say "brrrrrrr" and drop tone at end.