How is the score of GridsearchCV calculated? Is the score a percentage? Does this mean higher is a better?
The score is based on the scorer defined in the scoring argument. Meaning, the scorer can be any of the default metrics, such as precision, accuracy or F1-score (e.g., this); or a custom scorer. For a scorer (by convention), higher value is better. The value is not necessarily a percentage, but is often normalized between 0 and 1.