Mark all messages as read in Gmail Using Gmail version 86.0.04240.198 on my Samsung Galaxy A30s with Android 10.
From the documentation we can read

You can't mark all messages as read from the Gmail app.

I know this is a task that can be done in a computer but that's not an option for me ATM.
I tried going to gmail.com and from there activate &quot;Desktop mode&quot; but I was getting a responsive view with less functionality.
The way I've solve it was to

Open Google Chrome.
Activate Desktop mode.
Navigate to Google.com.
Search for Gmail.



Click in the Gmail. In my case was the first item in the result list.
Once in Gmail, click in the select all button which then opens up a message saying to select all (note that the image from this step was taken after the one in step 7, that's why you don't see a number of unread messages in the Inbox which shows up slightly in red)



Click in that message to select all, then go to the three dots and &quot;Mark as read&quot;. Once it's done, you'll get a snack bar in the bottom right informing it's finished.


Now there's no unread messages

Note: in my case then going to the Gmail app I could see that the emails were all marked as read even though the browser wasn't showing right away.