Nobody cares when touch keyboards predict the wrong pronouns, so why would the same service in an email client be any different?
A larger issue is that touch keyboards predict using single words and pretty quickly spiral into Markov chain incoherence if you let them draft whole messages. Smart Compose tries to offer syntactic and even semantic understanding. If I type "The investor is here, do you want to meet", my phone keyboard is working off 'meet', and will probably suggest 'up'. Smart Compose is working off 'investor' also, and apparently arrived at 'him'. (It's not clear to me from the article if this was actually tracked to specific biases, or if the predictions just adopted the default masculine.)
But most of all, I'm guessing Google's core motivation is explained by the offhand comment that Smart Reply uses the same logic. A gendered pronoun suggestion probably isn't going to spark a scandal, but a feature that encourages you to send Google-drafted text unedited might be subject to a lot more scrutiny.
Google doesn't actually care either. I hate call it virtue signaling but that's totally what it is even though that term is completely worn out at this point.