One month may not matter too much but a year certainly would. I would recommend just throwing it away and getting new. It's not like it's overly expensive.
so basically JJ.. the longer it's out of date the more likely it is to cause skin irritation. Why do you want a large quantity of our of date pain relief gel?
Don't have gel but ibuprofen that was out of date by a long way. I think it was fractionally less effective but when there's no new stuff in the medical cabinet... it does fine.