StarControl II (these days called Ur-Quan Masters) has quite a lot of grinding in terms of scanning planets for minerals, life forms and energy beacons. Gas giants seem to always produce empty scans. Is there a reason to ever visit\scan gas giants in StarControl II?
In general, no, there is no reason to visit gas giants. There are no resources to be found on them.
However, there is one important exception: One single gas giant in the galaxy should be visited as it holds something special.
You can't land on them, therefore you can't collect anything from them. The best you can hope for is a moon with something worth collecting.