Oats are very calorie dense. Try this:
2c Oats
2c Skim Milk
2 scoops Whey
Some ice.
Splenda, Cinnamon, & Honey to taste
Roughly 1000 cals and fits in a quart bottle.
If you want even more cals, add a little flax or extra light olive oil, ff cottage cheese, egg whites, &/or frozen berries.
I really don't see the point in buying commercial gainers since they're usually way overpriced. If you're going to drink a bunch of sugar, order a big tub of maltodextrin, aka "Carbo Gain" from NOW foods or another company. Mix it w/ your choice of whey. Add milk and anything else you want.
My little secret: Get skim chocolate milk at your grocery store. Make sure you get the stuff made w/ sugar, not splenda. It's 150 cals per cup. Just mix 2 cups w/ 1-2 scoops of whey. Eat a banana or bagel w/ it. It's an extremely cheap & easy 5-700 cals.