The problem with 'G-rekeningen' is that it is a lot of effort on the side of the bank to facilitate these kinds of accounts and that the future of this system is uncertain.
Since 2013 the new 'Ontvangersdeposito' is due to take effect and although it has been delayed several times, the plans are still there. The G-rekeningen system is end of life.
I would not like bunq to set up an infra structure to facilitate G-rekeningen when it is uncertain how long that system will last. They better wait for a new system to invest in.