in the wikipedia article about GBBS is written that there had been a GBBS System that supported ProTerm - https://en.wikipedia.org/wiki/GBBS .
I would like to set up a dial-up BBS with GBBS which is able to connect to a Raspberry PI.
GBBS connection with a modem in slot 4 (just dial-up)
GBBS connects to the raspberry pi in slot 2
and sends this via the modem to the Apple II which dialed-up the GBBS System. So this Apple II can use the internet connection from the raspberry pi(running Raspple).