The great Renaissance and late Medieval composers seem particularly underppreciated on this site (and to be honest among "classical" music fans in general) in proportion to their achievements. I suspect that most of this is simply because people haven't encountered their music, or listened to it with much attention if they have, and may on the basis of their very limited listening formed a false conclusion about it being all more or less the same and not really worthy of further attention. I just added Gibbons to the "Greatest Composers" list myself; it seems that the only time he was mentioned was by the OP, in the context of composers who no one would be expected to actually vote for (in order to illustrate how the voting system worked!).
I will admit to not having heard a great deal of Gibbons yet myself, but he is definitely near the top of my list of composers to explore more fully in future. It's ironic that he now appears so little regarded by most people. Whenever I think of Gibbons I remember this beautiful tribute paid by William Byrd after his untimely death, and recall that this is a composer who was held in such high esteem by his contemporaries that one of the greatest of them all had no hesitation in saying that music, itself, had died with him:
