JETSET GILLES: ‘Tis the season when collections have ended and designers are racking up frequent flier miles as they jet from trunk shows to fittings to investor meetings. This week J. Mendel’s Gilles Mendel will be in Los Angeles to open his first Saks Fifth Avenue shop-in-shop (the chairs are mink, the rug, Mongolian lamb) and show the spring collection at a luncheon benefitting the Aviva Family and Children’s Services. Supporters of Mendel and the charity, such as Colleen Bell, Jamie Tisch and Brooke Shields are expected. Then its back to New York for business appointments, and then off again to Paris and Milan for design work. Let the fur fly.