It's been a while since I tuned my GZ OLED but you're normally offered the chance to choose tuners when going through the initial set-up procedure. If you only tuned the Freeview tuner initially, then to add the Freesat channels, I think you have to go into the Freeview tuning menu and there should be an option to 'Add TV Signal'. If you haven't done any tuning at all, then just look for the same option in the TV's setup/tuning menus.
I have my GZ connected to an old Sky dish that the previous owners of my house left when I moved in in the mid 2000s and it works fine. As JayCee mentioned, I believe you have to change the LNB if you have a newer dish that has previously been used for Sky Q.
I have noticed recently though, that some equipment (like the latest Freesat PVRs) will work with Sky Q LNBs but I doubt this compatibility extends to Panasonic TVs.
One thing to note is that Panasonic TVs haven't officially supported Freesat for quite a while now so program information on your JZ will be limited to Now & Next info - there's no EPG, which isn't ideal if it's something you plan to use a lot for your TV viewing.