[Bug 42297] Gecko does not install itself in 2.0rc6

https://bugs.winehq.org/show_bug.cgi?id=42297

--- Comment #5 from Bruno Jesus  ---
They are stored in a folder inside your home folder, at least for me in Debian:

bruno@Note:~$ ls ~/.cache/wine
wine_gecko-1.8-x86.msi   wine_gecko-2.40-x86.msi  wine-mono-4.5.6.msi
wine_gecko-2.21-x86.msi  wine_gecko-2.47-x86.msi  wine-mono-4.6.3.msi
wine_gecko-2.24-x86.msi  wine-mono-0.0.8.msi      wine-mono-4.6.4.msi
wine_gecko-2.34-x86.msi  wine-mono-4.5.2.msi
wine_gecko-2.36-x86.msi  wine-mono-4.5.4.msi

Then when a new prefix is created wine checks if you already have the current
gecko/mono version downloaded, if yes then it will auto install, if not it will
prompt for download and store in this folder.

Did you check wine uninstaller?