Mavrovouni is certainly the longest and most crowded beach close to Gythio. It is a great strech of soft sand where you can lay on and feel relaxed under the sun! I loved this beach, you could lay for hours under the hot sun and hear nothing but the children playing and the sea waves... If you want to do some sports, there are bananas and water skiing clubs in Mavrovouni. Plus the beach is very close to the town of Gythion to go for a drink in the evening. I guess if you want to find an accommodation in Gythion, better stay in Mavrovouni to have the sea close by.