D has been in development since 1999, so it's more complete than Zig and its ecosystem is bigger. It is not just “Zig with GC”. D has:

- Multiple production-ready compilers. (Zig does not claim to be production-ready yet.) https://wiki.dlang.org/Compilers

- A more stable API. (Zig's authors rightly expect it to change before 1.0.)

- A package manager (dub) and repository: https://code.dlang.org/ (Zig will get support for these but doesn't even have an HTTP client needed to back package fetching yet.)

- An HTTP server and client with SSL/TLS support. (Zig is too new to have these, although community libs are starting to spring up to fill the gaps.)

- Better documentation, including a very accessible tour at https://tour.dlang.org/ and books such as http://erdani.org/tdpl/.

Zig may get all of these in time but D had quite the head start. D is a good language and I wish it got more attention.