Hello, and sorry the late reply.
GTM is built for Joomla's plugin framework, which is a completely
different environment than phpBB.
I'm quite familiar with phpBB (phpBB3 that is) and the way to
build Extensions is far less advanced than Joomla's framework.
In phpBB3, i doubt whether it's possible at all to get something
like GTM to work without changing the Core source codes...
...so i'm afraid the short answer is: No, GTM only works inside Joomla.