The physicists answer: there could be such a measure (it exists for example for the insulation of buildings), but it would in the case of clothing depend on so many factors that it would be close to useless. Factors that have an influence here are things like physical activity, wind, humidity or personal disposition.
To begin with the last one: There are people who get cold hands even if they just go out to collect their letters and newspaper from the letterbox and such that can engage in a two hour snowball fight without wearing gloves. And, this can change to some extent on a day to day basis. That already sets quite a wide base for a temperature range.
Then there's physical activity. Let's compare it with other pieces of clothing: While a good base layer shirt and a wind breaker jacket may be a combination totally suitable for an hour run in the winter, the same setup in the same weather would let you freeze within minutes while standing at the bus stop waiting for the bus. With gloves its not different. Need an example? You can see many soccer players play without gloves even in deepest winter with temperatures well below freezing – that's only possible since they are physically active and the blood flow heats their hands.
With wind and water you have another two factors that can change a lot. I have a pair of thin fleece gloves that are really really nice and moderately warm in normal weather. However, once it gets a bit windy or they get wet, they seem to loose all their insulation. Now you might argue that your gloves won't get wet in sub-freezing temperatures. However, consider some light climbing or scrambling over rocky terrain in the mountains, where the sun has molten some snow or ice to slush. Sooner or later you will touch such a chunk of slush and bang!, your gloves are wet and possibly your insulation is gone.
To conclude, there are too many factors that influence the insulation itself as well as the perception of cold that it is quite impossible to deduce a measure of insulation that is valid for more than a dozen of people and being misleading for a wide majority.
Now you might argue why then there is such a temperature rating for sleeping bags or the unit Tog for duvets.
Well, things are a bit easier here, since some of the aspects above are not present with sleeping bags or less pronounced. First of all, you normally do everything to keep your sleeping bag away from water and build your camp site in a way to keep wind away – hence those external influences are normally eliminated to some extent. Then, people are (usually) not physically active in a sleeping bag, so you can assume less variation in internal heating.
What stays is the cold vs. warm people problem. This is addressed in nearly every text about sleeping bag temperature ratings by saying that these ratings are generated using a (more or less) standardized test procedure and telling everyone that their actual comfort perception may be shifted with respect to that rating.