If the cost price of $12$ pens is equal to the selling price of $8$ pens, the gain percentage is:

Let the cost price ($\text{CP}$) of $1$ pen be $\text{Rs. }1$.
Therefore, the $\text{CP}$ of $12$ pens $=\text{Rs. }12$, and the $\text{CP}$ of $8$ pens $=\text{Rs. }8$.

According to the question:

$$\text{Selling Price (SP) of }8\text{ pens}=\text{CP of }12\text{ pens}=\text{Rs. }12$$

Now, calculating profit on $8$ pens:

$$\text{Gain}=\text{SP}-\text{CP}=12-8=\text{Rs. }4$$

$$\text{Gain Percentage}=\left(\frac{\text{Gain}}{\text{CP}}\right)\times 100\%=\left(\frac{4}{8}\right)\times 100\%=50\%$$