Vivaldi and Brave are not listed as supported on Gmail's website..
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Doesn't mean they won't work, but does mean you may need to find work arounds to get it to work..
In all cases, the browser and device must have not only cookies on and accepted but JavaScript must be enabled per Gmails help page.
Found a help page fore Vivaldi that may prove helpful to you..
HEREI believe Vivaldi is a Chromium derivative like MS Edge and Brave is a firefox derivative.. The problem is many websites go fishing for very specific minimum browser versions and often blocks or rejects browser versions that do not fall into the supported list.. In those case one may have to resort to faking out the website to believe it is dealing with a high enough version of Chrome, Edge, IE, FF..