Copy the .desktop file to your local applications direcory:

$ cp /usr/share/applications/org.gnome.Terminal.desktop ~/.local/share/applications/

Edit ~/.local/share/applications/org.gnome.Terminal.desktop

Add new-tmux-session to Actions:

Actions=new-window;preferences;new-tmux-session;

Add the new action:

[Desktop Action new-tmux-session]
Name=New tmux Session
Exec=gnome-terminal --command tmux new