Default values for the NULL arguments and further model details can be
found in the source link below.
In calls to varimp for EarthModel, argument
metric may be specified as "gcv" (default) for the generalized
cross-validation decrease over all subsets that include each predictor, as
"rss" for the residual sums of squares decrease, or as
"nsubsets" for the number of model subsets that include each
predictor. Variable importance is automatically scaled to range from 0 to
100. To obtain unscaled importance values, set scale = FALSE. See
example below.