I did a work around before, but this will be dangerous if the gw and portal is not compatible.
ls /opt/SecureSpan/Controller/lib/layer7-gateway-server*
you should find a file with a current version number of your gateway, for example,
/opt/SecureSpan/Controller/lib/layer7-gateway-server-9.1.00.jar
The install file will verify this version number, you can create a link to this file with an acceptable version number, for example, portal 3.5 accepts gw 9.0
ln -s /opt/SecureSpan/Controller/lib/layer7-gateway-server-9.1.00.jar /opt/SecureSpan/Controller/lib/layer7-gateway-server-9.0.00.jar
run ls command again, you will see 2 jar file with different version number.
install again, it will continue.
Again, this could be dangerous and only if you can't wait for Dustin
Never do this on prod env