Hi there Kman!
Fitday is has calculations built in for weight loss only, I'm afraid. It currently doesn't allow you to set a goal weight for gaining.
The reason for this, as I understand it, is that the calculations used for safely gaining weight are different than those that are used for safely losing weight; since Fitday has the weight-loss calculations programmed in, using it to gain weight would be inadvisable.
This thread might have some more information for you:
Female, 31 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 9/19/14 = 176.6 (net: -38.6)
Last mini-goal: 180 lbs. REACHED! Reward - new sports bra
Current min-goal: 175 lbs.
Next large goal: 165 lbs. by 12/25/2014
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."