Gluetun works great for other docker containers. For regular apps Vopono is another great solution, which makes it easy to for example run another instance of a browser in a separate namespace.

You might want to investigate vopono which allows specific applications to run in a separate network space. This you could for example run Firefox or qbittorrent in a separate virtual network that can only communicate via Mullvad VPN tunnel but not see anything outside it. This is great for desktop use. Another great option is gluetun which allows other docker containers to be bound to a VPN tunnel.