I am also experiencing this with one of my domains. Routing to gmail doesn’t work, but if I change the route to my old college address (an exchange email), it forwards.
The email bounces with this error:
Remote Server returned ‘521 5.3.0 Upstream error, please check [link blocked] for possible reasons why. Us6Wosh66ecE’
It looks like the rest of my domains are forwarding to the gmail account alright, it’s just one not working as far as I can tell.