Environment should be a space-separated list instead of comma-separated. No need for double quoting when the values don't contain spaces.
@@ -14,7 +14,7 @@ Group=git
WorkingDirectory=/home/git/gogs
ExecStart=/home/git/gogs/gogs web
Restart=always
-Environment="USER=git","HOME=/home/git"
+Environment=USER=git HOME=/home/git
[Install]
WantedBy=multi-user.target