Given \( a, c, b \) are in GP, so \( c^2 = ab \).
The area of the triangle formed by the line \( ax + by + c = 0 \) and the coordinate axes can be found using the formula for the area of a triangle formed by two lines intersecting the axes at \( x = \frac{-c}{a} \) and \( y = \frac{-c}{b} \).
The area of the triangle \( AOB \) is:
\[
{Area} = \frac{1}{2} \times \left( \frac{-c}{b} \right) \times \left( \frac{-c}{a} \right) = \frac{1}{2} \times \frac{c^2}{ab} = \frac{1}{2} \times \frac{c^2}{ab} = \frac{1}{2} { (using } c^2 = ab).
\]
Thus, the area is \( \frac{1}{2} \).