At one point about two years ago I went on a weight gain diet (this was well before I knew I had cancer) and would eat 6 meals a day, three of which were protein-based drinks. Whey Protein Isolate is your best bet digestion-wise, it's the easiest and won't make you feel bloated. It's almost pure protein with very little carbs. Add 2 tablespoons of oil (olive, flaxseed, etc) and you've got another 260 calories and 28 grams of fat added to it, for a total of about 500 calories. 3 of those and 3 other 400-500 cal meals is what it takes to gain weight with a high metabolism, but it did work for me as I went from about 174 to 195 in about 10 months.

I plan to use this logic when dealing with my cancer as I'm just about to start my journey through treatment. I realize it's gonna be near impossible to do this and I'll most likely end up with the PEG tube as well, but months down the road when I can eat and handle food without vomiting that's how I hope to put weight back on that I lose.

PS. Creatine adds weight by causing water retention so I'm not sure that's your best yet. It's generally only useful if you're working out a lot.

Also, those GNC mixtures with 1850 calories are very hard to digest. I've tried. They put you in the bathroom almost immediately upon ingesting.