If App Store is telling you that you need to pay for it, that usually means you are not logged in using the same Apple ID that you used when you first purchased it.
Check your Purchase History in the App Store to see if you can find that app. If you don't see it, that means you did not purchase it with your current Apple ID. I just checked my own and see purchases back to 2008, which is when the App Store was introduced.
There is another less likely possibility. The developer of Geometry Dash may have pulled the app out of the App Store, and then released another product under the same name with more features. If that is the case, then your old purchase would not entitle you to the new version.