In case this is useful to others with the same problem...
Did you checked whether the Force Secure Redirect setting was set True.
If you restored a gateway backup from another server the SSL certs supporting SSL / TLS on that server would not have migrated with the Ignition backup and you might be in the situation where SSL /TLS is not enabled and Force Secure Redirect setting is locked as True, with the ability to change it greyed out in thw gateway UI.
If that is the case you may need to edit gaterway.xml directly changing
<entry key="gateway.forceSecureRedirect">true</entry>
to
<entry key="gateway.forceSecureRedirect">false</entry>
See: Java error when starting the designer - #4 by tyler.bennett