Example with a Gmail web app, based on Brave Browser.
Find the latest release of Brave https://github.com/brave/brave-browser/releases.
Extract the content of the brave-browser-nightly-<version>-linux-amd64.zip archive, containing the brave binary and its librairies into
gmail/usr/local/gmail/.
dpkg-deb --build gmail
Will build whatever is in the gmail/ dir into your fs That's why I put files under a directory, otherwise the .git is included.
sudo apt install ./gmail.deb
Tada!