I found this short review of the three major units with a climb feature.
ClimbPro vs. Climber vs. Summit Compared - Cyclists Hub
Anyone have something to add from their experience with this feature?
Do map coordinate calculations differ much from a data field estimating current gradient during a climb?
I’ve got a Garmin 840 and the current gradient on ClimbPro seems reasonable, not that it gets more than a quick glance when I can tell the road pitches above 10%. Top of climb has never been off by more than about 5 seconds or so.
They are largely similar across the 3 units. If you are demanding pinpoint location accuracy to exactly deliver efforts according to highly accurate % gradient then I think you have unreasonable expectations from GPS. Much better off looking at the road. When I graduated from college a GPS receiver was the size of a refrigerator 
My rough recollection is that Garmin was first, then Hammerhead one-upped Garmin, then Wahoo came out, then Garmin added ClimbPro without a route loaded. So no surprise they are all pretty similar.
I have found that the realtime Hammerhead gradients are superior to the gradient info available from any mapping tools. One exception: I did a ride on Maui with a lot of overhanging, red (iron) cliffs and missed about half the predicted elevation, so it’s not perfect.
Fortunately the first handheld GPS receivers launched in 1989.When SA was turned off around 2000 then the offerings just got better and better.
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They all appear to use map data to show gradient in steps to indicate changes in slope.
If not particularly accurate is the value more in telling you how far you have to go on a climb and which sections are steeper than others?
Is there an online map site that allows you set start/stop points and display % slope for that section? Or drag a point along a road and slope is displayed.
Strava lets you create segments on any routex you have ridden but it’s a tedious process and you must save the segment to see the %.