I think GSMArena might even be being optimistic: I've browsed Sammobile.com and can't find anything newer than Android 7.0 for any S6 model. So this does seem very strange: as others have said, unless you are using custom software there's no way a Samsung Galaxy S6 would have Android 10, but equally you would know for sure if you had custom software (if you've had the phone for 5 years, i.e. longer than Android 10 has been out, the only way you could have an Android 10 ROM would be if you had installed it yourself). So there is something funny here.
Of course that doesn't explain why email notifications should stop. And if it just stopped working after you changed your password then it's clear where the problem lies. One thought: you say you logged out of and into the account in order to update the password, but did you update the password in Spike? A third party email app most likely does its own connection to the email server, so updating the password in the Google account on the phone probably wouldn't update it in a third party app: I changed my GMail password a few days ago and had to enter the new password into both of the email apps I have on my phone (in the apps' settings). So it might be worth checking that - if you can get that one working that would be something.
As for the GMail app (which I don't use myself), as a pre-installed app you can't uninstall it but you can uninstall all updates, after which you will see the latest version as an update in the Play Store which you can then install. Or you can clear the app's data completely and then re-connect it to the account (of course if you have more than one email account in the app that will clear them all). Or I guess just remove the account from the GMail app and then re-add it. But this is all just general guesswork: I got a bad impression of the GMail app back in 2010 and have never bothered using it since (I use third party apps for almost everything anyway, as I can usually find an alternative that I prefer).