Well, the easiest thing to try is simply adding an "s" to http and see if you can reach the service. If you can, then you should be good to go. If you can't, you'll need to speak with the admin of the GIS Server site or your IT staff to enable HTTPS on whatever is the front end of the GIS Server, (IIS and the web adaptor, reverse proxy, etc). Here's some documentation that may help.