The purpose of Gluetun is to isolate your VPN into a container that you can then bind as the network provider of other containers. However, it's a documented problem [0] that performance isn't anywhere near as good as running the VPN natively, especially for Wireguard. At the minimum this can be related on some systems to the container being unable to load the kernel-space driver and having to rely on lower performing user-space driver, but even with a pre-loaded kernel-space driver performance testing is reporting significant lower values than the host. Presumably this project is to try and resolve that issue.
It's the first time I hear about Gluetun, thank you for sharing that!
From a very quick first impression it seems like it solves a different problem than dwgd.
With Gluetun you are creating a container that has access to various VPN providers (and exposing ports where proxies listen apparently); with dwgd you are giving a container the capability to route packets toward a WireGuard tunnel (you are essentially just giving the container the "ownership" of the WireGuard interface).
[0] https://github.com/qdm12/gluetun/wiki/Wireguard#performance