At least one of the major operators offered a trip walking the Samarian Gorge when we stayed at the Aphrodite Beach Club hotel in Gouves a couple of years ago. My OH did it, and found the timing was good. You don't have to stay in the hotel to meet the rep and book a trip. Do remember to bring SUITABLE walking shoes or boots and plenty of water, plus fare for the boat back to Chora Sfakion and something to eat ... it's a long day away!