I've searched and not found any good resources on how to configure KMail post-May 2022 for use with Gmail after having experienced cryptic messages in KMail about "rejected wallet access", but I have managed to do so, so I put this here for anyone else's benefit.

From Gmail itself, you have to have two factor verification turned on. You get to that by clicking on your personal icon at the upper right corner of the Gmail in-browser view and selecting 'Manage you Google Account'.

Select 'Security' from the menu at left, then scroll down to 'Signing in to Google' where you have to enable 2-Step Verification.

Once you've gone through that somewhat tedious task and promised your last born to Google, come back to the same place where a new item will have appeared: 'App passwords'. Select that.

Then create an app password for KMail: select 'Mail' from the 'Select app' drop-down menu and in 'Select device' choose 'Other' and give it a custom name, e.g. "KMail".

It then generates a 16 character passcode that you can copy. And you do need to copy it. I copied mine into a special entry in KWalletmanager so not to lose it.

Now go to KMail and either create or modify a GMail account by using that passcode as the password. And, importantly, under the 'Advanced' tab in the IMAP account settings window (if you're not there already, click on the account and then the 'Modify' button in the 'Receiving' tab of the Accounts configuration window) you'll also have to change the 'Authentication' method to 'Clear text' from what is likely 'Gmail'. It's possible that 'LOGIN' or 'PLAIN' might work as well, but I have not tested them as 'Clear text' does work. As at the time of writing (July 2022, using KMail 5.19.3 in Kubuntu 21.04), the KMail account creation wizard is still creating Gmail account configurations with the authentication method set to 'Gmail', so you do have to go back and modify this one item.

How this actually counts as being more secure I'll leave it to those more familiar with the subject than me, but it does work.

Hope this helps someone.