If I wanted to gain weight using a calorie-counting tool such as the fitday log, I would find the calories I needed to maintain my present weight. Then, with daily food log information, I'd increase my caloric intake. If I gained weight, I would know I was increasing my calories enough. If I didn't, I would know I need to increase my calories more.
This depends on accurate food log entries. With fitday, you will also see how well-balanced your diet is with protein, carbohydrate, and fat grams.