This is how we all learn about all of this stuff. The GTA 5 update was planned for release. So if you wanted to, you could have selected the option to not load Steam on startup. Then when started steam and it tries to update, just click off that to close it out and then go to Steam and then to "offline only" mode. This will work for awhile, but eventually Social Club will want to connect again. At that point you can maybe try rebooting PC and see if you can go back into offline mode.
I tell you this, because Rockstar will continue to update GTA 5 and milk their online baby for several more years. Use this as a learning experience for next time.