Let the selling price be \( S \) and the cost price be \( C \).
Given that the gross profit is 26% of the selling price:
\[
S - C = 0.26S
\]
Rearrange to express \( C \) in terms of \( S \):
\[
C = S - 0.26S = 0.74S
\]
Now, we are given that 34% of cost price is:
\[
0.34C = x% \text{ of } S
\]
Substituting \( C = 0.74S \):
\[
0.34 \times 0.74S = x% \times S
\]
\[
0.2516S = x% \times S
\]
\[
x = 25.16%
\]
Thus, the required percentage is 25.16%.