This is a very interesting topic and I am glad it has been brought up. I think both Jack and Yugo are right (and wrong) to some extent.
I think Yugo is right in a sense that the knowledge gap exists, but he overvalues it a bit, and undervalues the speed. Jack, however, undervalues the knowledge gap.
Now beware - these percentages are basically coming just from my experience and estimation, and not some hard proof.
I would say that on a whole, I "know", in a sense that it does not take much time for me to think about it, about 60ish% of what an average 2700 knows. Of the remaining 40%, I would say about 30% I don't "know", but am able to figure out over the board (a very important factor, IMO, that neither of you two addressed directly), and only the remaining 10 or so percent are completely above my grasp, i.e. I would not figure it out despite given any amount of time (unless of course I improve as I am looking for it
). Out of the 30% that I don't "know", but can figure out, the majority consists of the opening stuff, i.e. I feel like this is where the biggest advantage of a GM lies over me.
The 30% that I don't know, but am able to figure out over the board, correlates exactly to what Jack is saying - since competitive games are played with a time limit, the fact that I have to spend a ton of time figuring out this 30% amounts to the largest part of the ELO rating gap. Crudely speaking, GM's don't need to spend time on that, therefore they have more "speed", therefore they are less likely to end up in time trouble vs me, therefore they win more often than not. If I would have an unlimited time to figure stuff out, I would say the ELO gap just based on difference in knowledge that is beyond my grasp, i.e. stuff that I cannot figure out at all, would consist of at most 100 ELO points.
I have three very recent examples of this, actually, even though my opponents weren't 2700, but all three were GMs, one above 2600, one just shy of it, and one 2530ish (but he won a tournament with massive performance, so I guess you could say that at least that tourney he was at least playing at that level). All three games were eerily similar. I knew less in the opening in all three games = I spent a lot of time having to figure it out = I figured it out for the most part, but was way behind on time = when approaching time trouble the evaluation was absolutely equal in all three games = I lost all three games in time trouble. So, I could hang with them "in chess" for sure, but could not hang with them "in speed".
So you could both say that I lost because of the knowledge gap (in these cases, mainly in the opening), and that I lost because of their speed (because I was able to "cover" the knowledge gap over the board, but had to spend time on it). So you are both kinda right