Weight gain /loss depends on lot of factors. You can have a check stuff physician about all are OK or not.
For such gain have wholesome meals, rich in protein n calories both. Like milk shakes, fruit smoothies, fruit custard etc. Have a heavy dinner with cereals n proteins.
Have lots of fluids throughout the day. Do exercises that suits your body.
The diagnoses are quite variable and may include reduced food intake, organic causes,or psychological disorders; however, among many older adults, no obvious medical cause can be identified. A variety of nonpharmacologic interventions may improve energy intake and lead to weight gain. In some cases, access to quality nutrition is the main issue. The most common approach to the treatment of weight loss among older adults is consumption of high-energy/high-protein oral supplements between meals as a means of increasing daily energy intake. The role for pharmacotherapy remains limited at present....Avoid junk foods,add extra calories to your diet,have 4 to 5 meals rather than large 3...add protein rich foods and healthy fats to diet for e.g..egg white...add fibre rich foods like green leafy veggies,carrots,Apple, etc. Add green tea to your diet .you have to eat more foods high in amino acids,calcium, zinc, iron, copper and magnesium...do regular exercise without failure......consult dietitian for further details