A shopkeeper buys 8 articles for Rs. 800. He sells 5/8 of them at a gain of 20% and the remaining at cost price. Find his total gain.

Let the total number of articles be 8 (to simplify calculations with the fraction $\frac{5}{8}$).
Let the cost price (CP) of each article be $\text{₹}100$.
The total cost price for all 8 articles is $8 \times \text{₹}100 = \text{₹}800$.

The shopkeeper sold $\frac{5}{8}$ of his articles at a gain of $20\%$.
Number of articles in this part = $\frac{5}{8} \times 8 = 5$ articles.
Cost price of these 5 articles = $5 \times \text{₹}100 = \text{₹}500$.
Selling price (SP) of these 5 articles = $\text{CP} \times (1 + \text{Gain Percentage})$
$\text{SP}_1 = \text{₹}500 \times (1 + \frac{20}{100}) = \text{₹}500 \times 1.20 = \text{₹}600$.

Remaining articles = Total articles - Articles in the first part = $8 - 5 = 3$ articles.
These remaining articles were sold at the cost price.
Cost price of these 3 articles = $3 \times \text{₹}100 = \text{₹}300$.
Selling price of these 3 articles = $\text{CP} = \text{₹}300$.
$\text{SP}_2 = \text{₹}300$.

Total selling price = $\text{SP}_1 + \text{SP}_2 = \text{₹}600 + \text{₹}300 = \text{₹}900$.
Total cost price = $\text{₹}800$.
Total gain = Total selling price - Total cost price = $\text{₹}900 - \text{₹}800 = \text{₹}100$.

Gain percentage = $\frac{\text{Total Gain}}{\text{Total Cost Price}} \times 100\%$

As a mixed fraction, $12.5\%$ is $12\frac{1}{2}\%$.

The final answer is \boxed{\text{12\frac{1}{2}\%}}.