Hello -- I'm sure will you receive a bunch of suggestions on this post. Some aspects and recommendations will be personal (and professional) preference based on past scar tissues doing similar CP upgrades.
Without going through the exact details, I strongly recommend splitting this into "phases". First phase is only upgrading Smartcenter instance. You do this successfully and let "dust settle" for period of time (days/weeks/etc) before moving on to gateways.
If on Vmware/HyperV/Nutanix, this is fantastic and makes the procedure VERY flexible from operational standpoint. If on physical hardware, you lose some flexibility but overall procedure very similar => "advanced upgrade".
you'll be doing what is called an "advanced upgrade" which means you'll build an entirely NEW instance in separate VM, install GAIA from scratch from ISO, run wizard, update to latest GA JUMBO, then IMPORT config via migration tools.
There are obvious sequence of events that have to happen -- example: turning OFF the OLD R80.30 instance before you install new instance and pick the same IP address. there is scenario where you can install new to different IP, import, and test access with SmartConsole and "cut over" IP when appropriate (turning off OLD and changing IP of NEW to production IP). The "cut over" is complete with a policy push to make gateways aware of new instance. Yes, SmartCenter can be newer version managing older gateway versions.
Second key recommendation during advance upgrade: insure the SmartCenter object name (in CP software Smartconsole) is same as GAIA hostname on new instance. Do NOT change this during upgrade (due to reasons beyond scope of this thread).
the gateway upgrades are relatively easy -- HERE.