Once you have uninstalled the ActiveGate it might take up to 72 hours for it to disappear from Dynatrace. For the url it looks like it can be edited and wiped out that way using the pencil icon beside it.
Hello @David N.
Thank you for your comments.
Cluster ActiveGate was uninstalled 2 months before. I tired to remove with the edit button but it doesn't go anywhere.
e.g. after manual removal, click on X sign brings back the actual URL and click on Right Symbol gives the message of malformed URL.