I had a quick look through the Q&A and found nothing covering this situation, which usually indicates that I am missing something in my reading of the rules, but here goes...
The sticking point is whether change of status from IN the gully to crest status in the advance phase constitutes "advance into an Open Ground Hex" or "any other action" in the context of the concealment loss/gain table. One might argue that the unit is neither entering a new hex nor a new Location within a hex, and thus the first should not apply. However, it might be harder to argue that this is not an "action", per se, worthy of losing concealment, and it is certainly not covered in any exceptions.
On the other hand, if the rules indicate that such a change of status would strip concealment, we would be faced with a somewhat nonsensical situation whereby an advance from the bottom of the gully to its crest would lose concealment, whereas an advance from the bottom of the gully, past the crest, and across a few meters of open ground into a building or woods perhaps containing enemy troops, would not!
In the interests of common sense, I would advocate that in the above situation concealment is lost in the MPh, but retained in the APh, regardless of any enemy LOS INTO the gully. What the rules actually say, though...