If I want a gross margin of 25%, what percent should I mark up my product?

To achieve a gross margin or gross profit percentage of25%, you will need to mark up your product's cost by33.333%. 

Gross Margin States that in equation Sales = Cost + Profit  Sales will remain 100%

So if Gross margin is25% then the question should be:

At what percentage I should cost up my product?

Becuase Margin & Markup are two different concepts.

So,

100%=25% - Cost

and cost will be 75%

In concept of Markup:

Cost must be100%

So,

125% =100% +25%