Calculation of Cost Price and Selling Price:
To determine the gain or loss percentage, we first need to calculate the cost price and selling price of the eggs.

Given:
- Cost price of 12 eggs = Rs 8
- Selling price of 8 eggs = Rs 12

Let's calculate the cost price per egg and selling price per egg.

Cost Price per Egg:
To find the cost price per egg, we divide the cost price of 12 eggs by 12.
Cost price per egg = Rs 8 / 12 = Rs 0.67

Selling Price per Egg:
To find the selling price per egg, we divide the selling price of 8 eggs by 8.
Selling price per egg = Rs 12 / 8 = Rs 1.50

Calculation of Gain or Loss:
To determine the gain or loss, we compare the cost price per egg with the selling price per egg.

Gain:
If the selling price per egg is greater than the cost price per egg, there is a gain.

Loss:
If the selling price per egg is less than the cost price per egg, there is a loss.

Gain or Loss Percentage:
To calculate the gain or loss percentage, we use the following formula:

Gain or Loss Percentage = [(Selling Price - Cost Price) / Cost Price] * 100

Calculation:

Cost Price = Rs 0.67
Selling Price = Rs 1.50

Gain or Loss Percentage = [(1.50 - 0.67) / 0.67] * 100
= (0.83 / 0.67) * 100
= 1.2388 * 100
= 123.88%

Conclusion:
The grocer has a gain of 123.88%.

Explanation:
The grocer buys 12 eggs for Rs 8, which means the cost price of each egg is Rs 0.67. He sells 8 eggs for Rs 12, which means the selling price of each egg is Rs 1.50.

To find the gain or loss percentage, we use the formula [(Selling Price - Cost Price) / Cost Price] * 100. Plugging in the values, we get a gain of 123.88%.

This implies that the grocer is making a profit of 123.88% on the eggs he sells.