RobertK now I know Budgie is based on Gnome but when you download Solus Budgie can you switch to Gnome on the fly without having to install the DE separately?
No. It's not based on GNOME (Shell). It uses very specific libraries and binaries developed by the GNOME Foundation, such as libwnck, gnome-settings-daemon, GTK3, and Mutter. Its default login screen is lightdm and slick-greeter, neither of which is developed by GNOME. It's lock screen is a fork of gnome-screensaver, which GNOME doesn't even use anymore.
That person just doesn't know what they're talking about.
You can install gnome-shell, gdm, etc. separately and log into one or the other, but installing Budgie does not install any of that. We have documentation in the Help Center on this.