How do you know that's what he was told? Because he said so in an interview?
That's not the point. The point is that thinking otherwise is highly non-parsimonious.
If the medical staff told Gauthier that Markov had little chance to ever play again, it makes little sense for Gauthier to sign him for 3 years. It's not like he (and his staff) didn't understood the risk behind the decision. The only logical reason why he would sign him for three years was because he got information saying he stand a very good chance of coming back.
It is way more parsimonious to believe that the medical staff got it wrong (because that happen all the time) than to believe that Gauthier is so stupid he didn't listen to his medical staff. The first scenario is common. The second is hard to explain. We would need to explain how someone so stupid could even end up a GM for a NHL club, let alone get any kind of management job, without this very high stupidity being exposed early in his career. It just doesn't happen in real life. Or, at least, it is much much much much rarer than medical staff being wrong.
For the record, I think he'll be back close to the trade deadline, he'll play OK for the rest of the season considering he's been inactive for so long, and then he'll be traded during the offseason for a younger guy on D and maybe picks.