I mean, irrespective of who's better, Dumba was an RFA and Gardiner would be a UFA. Can't compare 1:1 like that at all. FWIW I think Gardiner would command at least $0.5M more than that on the open market. For better or worse points are a premium and he scores a lot of them.
I am not saying Gardiner wouldn't make a nice pay day on the open market because that's the nature of things and relative other signings he's "worth" that. But RFAs are there so teams can control young stars for longer (ie afford to pay them less than market value) and Dumba IS already a tier above Gardiner at that price, and very likely will get better considering he's improving every year here so far and is only 24.