I have a lot of sympathy for boards that block gmail addresses.
The reasoning is likely to be the same as blocking hotmail and yahoo and other free email accounts.
Unlike email acccounts directly with ISPs, the board owner cannot easily trace them back to a person.
Which means the freebie accounts get used by board spammers, doing URL drops and other extra-to-TOS activities.
The downside for the spammer is, at worst, the loss of a free email account.
Of course, blocking freebie email addresses does not alone stop board spammers. But security is build in layers, and for some boards, it is a necessary layer.
Which is a reflection on the sad state of the Internet.