Neither can play the role well at all, as neither of them are that position, that's were we disagree.
We either dont agree on the definition of a number 6 or don't agree on gana and garners abilities.
A number 6 to me isn't just someone who wins the ball back like what gana does, he pushes up and hunts for the ball and often can win it quite high up the pitch.
A number 6 would stay deeper and offer protection to the back 4, especially the fullbacks when they push on the number 6 would be filling that position.
Neither gana or garner do all of this, and know one at Everton as since barry
Sure, maybe both of them cannot play the role that you speak of and we probably disagree on the role itself. To me, your description of the role seems to be something that all of our midfielders seem to be tasked with under the pretext of being box-to-box midfielders. We are probably talking over each other cause of what we probably believe a role in football is supposed to be, I believe in a more systems approach compared to agentic or individualistic ideas of playing. I doubt many modern teams have No.6s who carry the burden of all of the actions you have mentioned. In today's game, I feel Barry would just be an inverted wingback (see similarities to Milner), Keane would be branded mediocre, and Viera would be openly scoffed at. Every role needs to be complemented by the tactics of the team.
For the sake of understanding, In the classic 'Dyche' style, suppose that the midfield unit is tasked with the roles of a 6-8-10 or a 4-8-10. Good midfielders should be able to play at least two of the three roles when covering for each other and similarly, the 10 has to be able to do the role of the No.9 when needed. Consider that in a narrow setup, the wide players stay and you have those overlapping runs and such between wingbacks and the wingers and such, and one of the midfielders drops in to cover for the CB or the WB depending on whether both WBs are up the pitch for the attack or if one of them move up as per the tactics of the team. But even then, the other midfielders need to share the responsibilities for the 10 and 8 along with the 6 because the '6' is now either a 5 or a 2/3. We then need to then consider the 7s and 11s and 2s and 3s. In my opinion, although I appreciate that all of this is still very very rudimentary and we must acknowledge that it is at best league one level, any manager that sets up a team without doing their homework on the opposition and their tactics or trusts in cookie-cutter roles as a sign of player suitability does not deserve to manage in the premier league. We need specific tactics for specific players notwithstanding the formation of the team and this requires an understanding of the player's game itself, yeah. We can't be edifying certain roles in the first team such that replacements are considered to just mirror the first team player's game, especially for players like Doucoure, Keane, and to a certain extent DCL who are all unique players.
Also personally, I prefer my 6 to be both a 6 and a 4 (in the midfield sense), rather than a 6/5 or 6/2 player, and a 6/8 isn't that bad but we need two of them together.