To find the total amount Griselda spends on getting her nails done before the tip, we can use the information given about the tip percentage.
The total amount after including the tip is $26.55, and we know that she leaves an 18% tip. This means that the total amount after the tip represents 118% of the original cost (100% for the service + 18% for the tip).
Let's denote the total cost before the tip as \( x \). The equation representing the total amount after the tip is:
\[ x + 0.18x = 26.55 \]
This can also be expressed as:
\[ 1.18x = 26.55 \]
To find \( x \), we need to divide both sides by 1.18:
\[ x = \frac{26.55}{1.18} \]
Now, calculating that value:
\[ x \approx \frac{26.55}{1.18} \approx 22.49 \]
So, the total amount Griselda spent on getting her nails done before the tip was approximately:
\[ \boxed{22.49} \]