Yikes. But also how unusual. I just checked in Gmail on my iPhone and not only does it have a third option of Safari, it has a fourth option of "Default browser app".
It seems (?) to be a bug but it makes me wonder how widespread the bug is, how often it's triggered. But that is very not cool. Thanks for the screenshot.
On my iphone the GMail app seems to frequently "forget" that Safari exists and I'm prompted to install Chrome when I tap a link.
https://i.redd.it/tg2yj98o5ao51.jpg (Obtenir means Get/Install).