When using gitlab with implicit grant, you need to disable

the netlify-identity-widget in gatsby-config.js.

gatsby-config.js

@@ -14,6 +14,7 @@ module.exports = {
     {
       resolve: `gatsby-plugin-netlify-cms`,
       options: {
+        enableIdentityWidget: false,
         modulePath: `${__dirname}/src/cms/cms.js`,
       }
     },