To find the gain percent, we can follow these steps:

1. Define Variables:
- Let the cost price (CP) of 1 pen be $C$.
- Let the selling price (SP) of 1 pen be $S$.

2. Use the Given Information:
- The cost price of 12 pens is equal to the selling price of 8 pens.
$$12C=8S$$
- Solve for $S$ in terms of $C$:
$$S=\frac{12C}{8}=\frac{3C}{2}$$

3. Calculate the Gain per Pen:
- Gain per pen is $S-C$:
$$\text{Gain}=\frac{3C}{2}-C=\frac{3C-2C}{2}=\frac{C}{2}$$

4. Calculate the Gain Percent:
- Gain percent is calculated as:
$$\text{Gain Percent}=\left(\frac{\text{Gain}}{\text{CP}}\right)\times 100\mathrm{\%}$$
$$\text{Gain Percent}=\left(\frac{\frac{C}{2}}{C}\right)\times 100\mathrm{\%}=\frac{1}{2}\times 100\mathrm{\%}=50\mathrm{\%}$$


The gain percent is 50%.

The correct answer is D. 50%.