Gose is a better fielder right now and comes with a better arm, is a better baserunner, and Rasmus' bat is incredibly streaky and prone to tremendous cold streaks (which we saw last year). As of right now Rasmus is better, but the lack of development out of him means that Gose has closed the gap. Another stagnating season and Gose can catch up. Consider that in spite of putting up a .223/.303/.622 line, and striking out an abhorrent 31% of the time in his limited MLB action last year, Gose still managed a positive WAR (0.6 for 189 credited plate appearances). Comparatively, Rasmus finished with a 1.4 WAR in 625 credited PAs. In other words, Gose provided almost half of Rasmus' seasonal value last year in 1/3rd the playing time. And given the rate that Gose accumulated his WAR, if he had played as many PAs as Rasmus, he would've finished as a 2 WAR player.
I may be incorrect in applying WAR in that fashion, but the logic seems sound (WAR is a cumulative stat, which means you should be able to extrapolate it for given points if you figure out what the accumulation rate is)
Regarding the trade, Ike Davis doesn't count as a prospect, so if you believe the Dickey/Thole/Prospect reports, then Davis can't/shouldn't be the 3rd piece.
and on the value, I don't like it. The goal of the Blue Jays has been explicitly stated as creating a system in which we don't have a window to compete. Where we can do our best to keep it open perpetually by building a strong farm and shuffling in good prospects as they are ready and as we can move the pieces they are replacing out for more assets or just because of age/decline/desire to leave. Trading 2 of your top prospects for a guy who is on a ticking clock in terms of usefulness goes against that. If it were one prospect I wouldn't have as much problem. Probably less problem if it were Syndergaard as opposed to d'Arnaud (catchers who come with the skill package he does are ultra rare). But the price just strikes me as too high as it's reported.