LOS ANGELES -- Arden Group, parent of eight-store Gelson's and four-store Mayfair Markets here, reported increased earnings and lower sales for the first quarter ended April 1, 1995. Net income for the 13-week quarter rose 160.5% to $1.4 million, while sales fell 5.1% to $59.9 million. Same-store sales fell 1.4%. Sales for the prior year included $2.3 million from GPS Pool Supply, a wholly owned subsidiary that was sold in May 1994. Excluding that volume, sales for the quarter -- consisting of supermarkets only -- fell 1.4%. An Arden spokesman attributed the sales drop to competitive pressures and to the fact that Easter and Passover fell during the second quarter of the current year but the first quarter a year ago. He said the big earnings swing was due to nonoperating factors: a gain of $761,000 this year in the value of Arden's securities following stabilization of interest rates, compared with a loss of $893,000 a year ago when interest rates went up and the value of the securities dropped.