Porridge in and of itself is not going to make you gain OR lose weight.
It all comes down to what your total calories are at the end of the day that determines if you lose or gain weight.
Now, one thing I'll say about porridge is that it is very filling and relatively low in calories....it "sticks to your gut" as they say.
Considering that, if you make it a staple in your diet by eating it for breakfast and lunch, you will be able to feel full on fewer calories, and provided you don't gorge yourself on a lot of food for dinner, you are most likely going to be in a caloric deficit and lose weight.
How do you find out if this is going to be the case for you or not?
Well, you need to figure out the calories your body needs to maintain it's current weight first:
bodyweight in lbs x 15 ... So if you were 150 pounds, that would be 150x15 = 2,250 calories/day.
Now 1 cup of porridge is about 360 calories.
If you had that and only that for breakfast and lunch, you would have eaten a total of 720 calories.
If for dinner you eat say 500 calories worth of food, that would put you at 1,220 calories for the day.
2,250 - 1,220 = a 1,030 calorie deficit.
It takes 3,500 calories to burn 1 pound, so if you were this hypothetical 150 pound person, you would burn 1 pound in about 4 days eating porridge for breakfast and lunch (provided you didn't do anything crazy for dinner).
If you wanted to GAIN weight with porridge, you would have to have 3 times the amount of porridge I was suggesting, 3 cups for breakfast and 3 cups for lunch. That is 3 bowls for breakfast and 3 bowls for lunch (along with the 500 calorie dinner).
Now if you're looking to setup a proper diet, there's more info here..