How to open new window in Opera GX
In order to open a new window in Opera GX (as opposed to a new tab), while the previous window(s) are open, click on the Opera icon in your start menu (or shortcut in your taskbar).
Trying to open Opera GX again while it’s already open will open a second window entirely.
Alternatively, you can drag out a tab out of the Opera GX window, e.g. on a different monitor, which will also open a new window.