maintainers: add gdd

maintainers/maintainer-list.nix

···
     githubId = 81654;
     name = "Damjan Georgievski";
   };
+  gdd = {
+    email = "[e-mail]";
+    github = "gabriel-doriath-dohler";
+    githubId = 40209356;
+    name = "Gabriel Doriath Döhler";
+  };
   gdinh = {
     email = "[e-mail]";
     github = "gdinh";