You can add an "External tool link" within your Moodle course that takes students directly into Gradescope, without their needing a second login. Here is how to add that link:
Among the grid-like view of activities and resources (listed in alphabetical order), click on the "External tool" link:
For the "Activity name," you might want to type in "Gradescope Link." Then, click on the drop-down for "Preconfigured tool," and click on "Gradescope":
You will now see a "Gradescope Link" icon within that topic area (assuming that was the Activity name you entered). Students click on that link to go directly to Gradescope: