In a rectangle, opposite sides are equal in length. Given that \( FH = a + 25 \) and \( GI = 2a \), we know that:
\[ FH = GI \]
This means we can set up the equation:
\[ a + 25 = 2a \]
To solve for \( a \), we can rearrange the equation:
\[ 25 = 2a - a \]
This simplifies to:
\[ 25 = a \]
Thus, the value of \( a \) is:
\[ \boxed{25} \]