I have an MGB a roadster rather than a GT so I don't have any rear seats.
But the situation with classic cars is that if a car wasn't fitted with a certain feature from new and that was legal then you don't have, by law to have them retrofitted.
My B was made in 64 before seatbelts were compulsory but at some point (quite early on from the looks of it) some were fitted. As that has happened it would now be illegal for me to remove them.
But I digress. Legally no people can travel in the back without seatbelts - But I wouldn't do it - not more than a small trip around the block anyway.
Having kids bumping around in the back of a GT would be bad enough. Have a shunt and they'd become airborne!
I'm pretty sure you can fit rear seat belts to a GT I use the MGB hive for parts
http://www.mgbhive.co.uk/index.html give them a ring and ask them, they're very friendly and helpful