My mother has an iPad. She has a gmail account and she uses the Apple Mail app.
One of gmail’s strengths has always been its ability to flag spam and keep it out of the inbox. However, on her iPad, she gets so much spam into her inbox. I’m constantly having to delete it.
I have gone into gmail.com and looked for any kind of setting to minimize the spam but haven’t really seen anything of note.
There are 3 total accounts in the Mail.app. One is an iCloud account that almost has nothing and the other is a gmail account that is pretty new and gets just a few emails to it.
I have three gmail addresses that deliver to a single Inbox and I cannot remember the last time any spam got past the filters. They work so let see if we can get them to work for you.
When you are in gmail.com in your browser you can mark a message in your Inbox as spam by clicking the octogon symbol.
I realize I was just thinking half the reply to myself, sorry for that. My reason for asking about other accounts was to suggest that the spam might originate from one of these non-GMail accounts. It may not be obvious to see the offending source account once you are just managing email within Mail.app.