I think most TT's in GT's are now 30-35km. Unfortunately.
Bernal lost 1m37s to Alaphilippe over 27k on a terrain that should have been more suited to his strenghts in the Tour'19 which he won. So he should have been in very good form, probably as good as now, maybe better.
Decay should worsen as a TT gets longer and riders are more tired, so we can't simply extrapolate the opening ITT difference (which would be 73 seconds). At the end of a GT, the more natural TT'er should be in the advantage, and certainly Evenepoel. Why? Because his most defining trait as a TT'er is his low Cda, not his power output. Even when he's more tired at the end of a GT, and he will lose power just like anyone else, his aerodynamic efficiency should stay roughly the same. In short, he doesn't solely rely on his power output, or at least not as much as other riders do, to make a difference. Say Evenepoel has roughly the same power output as Bernal (which would probably not be that far off), but he is the much better TT'er, then the only explanation for that is that his build and TT position are much more efficient than Bernal's. Now say both lose 5% power on the TT bike due to fatigue over 3 weeks, for Bernal those 5% account for a lot more in terms of actual time since his power output is more determinative of his result, than in Evenepoel's case, who can also still fully rely on his low Cda even if he also loses 5% power. Furthermore, the slower both ride (due to the hypothetical 5% of power loss due to fatigue), the bigger the time gap will become. The slower they ride, the longer it takes to finish the TT, the bigger the time gap between both riders.
Theoretically and hypothetically. If Bernal's recovery over 3 weeks turns out to be superior, this entire exercise goes down the drain.
I think if Bernal wants to be sure he won't be caught (assuming both are still in good form and haven't shown signs of imploding) i think he should have a buffer of 2m30s. Even then a crash or puncture could decide otherwise. I'm not saying Bernal will lose 2m30s, but if i were him, i wouldn't feel comfortable with 2m or less. Don't forget 19 year old Evenepoel took 4-5 minutes out of Jungels, Martinez, Tratnik.. over 55k.