Put this folder under $XDG_CONFIG_HOME (ie. the variant file should end up
as ~/.config/xkb/symbols/gb).

Log out / back in, then select the new variant. Under GNOME, go in the
"Settings" tools, "Keyboard" page, in "Input Sources" the little "+",
"Add an Input Source" for "English (United Kingdom)", and find
"English (French accents on dead keys)".