I have never experienced this with Gmail, though I use ublock as you mentioned. I haven’t seen any through the gmail app on mobile either.
Email is an open protocol, there’s no reason to continue using an email client (web or otherwise) that displays ads. I highly recommend you use something like thunderbird if this is your experience.
Ads are pernicious no matter where they appear, but them being associated with any kind of personal communication is fucking nuts, I wish we wouldn’t normalize it.