I've checked that the Auto forwarding is working as expected.
I was going to do a quick response and say it's gmail that's not accepting mail from off-platform sources because my first test didn't get through to gmail. However, every subsequent test of off-platform mail incoming to talktalk and Auto forwarded has worked.
So, I cannot be definite as yet because there's been no bounceback error from gmail but I'd say it's gmail deferring delivery pending scanning for spam. I'll wait and see if my first test does eventually get received or bounced.