Solution:

Given:
- The milkman adds 100 ml of water to every 1 liter of milk.
- He claims to make a profit of 25%.

To find the actual gain percentage, we need to understand the process of diluting the milk with water and how it affects the cost and selling price of the milk.

Understanding the Dilution Process:
- The milkman adds 100 ml of water to every 1 liter of milk. This means the total volume of the mixture will be 1 liter + 100 ml = 1.1 liters.
- The ratio of milk to water in the mixture is 1 liter : 100 ml, which simplifies to 10:1.
- This means that for every 10 parts of the mixture, 1 part is water.

Calculating the Cost Price:
- Let's assume that the cost price of 1 liter of pure milk is 'C'.
- Since the milkman adds 100 ml of water to 1 liter of milk, the cost price of 1.1 liters of the mixture will be 'C'.
- So, the cost price of 1 liter of the mixture is 'C/1.1'.

Calculating the Selling Price:
- The milkman claims to make a profit of 25%. This means the selling price of 1 liter of the mixture is 125% of the cost price.
- The selling price of 1 liter of the mixture is (C/1.1) * 125/100 = (1.25C) / 1.1.

Calculating the Gain Percentage:
- The gain is the difference between the selling price and the cost price.
- Gain = Selling Price - Cost Price = [(1.25C) / 1.1] - (C/1.1) = (0.15C) / 1.1.
- The gain percentage is given by (Gain / Cost Price) * 100.
- Gain Percentage = [(0.15C) / 1.1] / (C/1.1) * 100 = (0.15 / 1) * 100 = 15%.

Conclusion:
The actual gain percentage is 15%, not 25% as claimed by the milkman.