The title says it all - is it based on webkit, gecko or something else?
Android default browser uses webkit Check out second paragraph : Wikipedia Webkit description
After reviewing some user-agent strings, all native Samsung browsers I saw use WebKit.
There are particular idiosyncrasies when working with this family browsers amongst devices, and a lot of weird behavior. According https://stackoverflow.com/a/19551809/489396 there is some "value added" by Samsung to the AOSP browser.