When I first switched from WPC to WPI, I threw up three times in a row, but now I have two servings a day and it's all good and I have a serving of WPI.
Really, in the end mass gaining is about maintaining a positive energy balance, a 2000-4000kJ deficit should do the trick; focussing on breaking up your meals so you can get more energy in(try avoiding to many low GI foods as this will keep you full, you don't want that).
Add a decent resistance training program and you'll put on mass in no time.
Deficit for mass gaining? Lol.
I don't want to gain mass anyway. Surgery left me with a couple of kegs of puppy fat I gotta lose, but I'm pretty happy being 5'11 and 85 odd kg after that