Was your +3 a 27 or a 278?
When you see the new vet, ask to have the vet check Gentle's teeth. I'm wondering if the swings in Gentle's numbers may be due to either old Lantus or a dental issue. If you opened a new vial 3 weeks ago, the Lantus should be OK. As for a dental, I was referring to a cleaning. Cats can develop tarter which causes inflammation. Any inflammation (or infection, for that matter) will affect BG.
As for the pens, I'd call around and see if there are outlets where you can buy a single pen at a time. Someone posted that either their local Sam's or Costco would sell them singly and you don't need to be a member to use the pharmacy. Some of my local university medical centers pharmacies will sell the pens singly, as well. If you have that information ahead of time for your vet appt., the vet can write the Rx with as many refills as you want, probably up to 12.