How to see the optimal cutoff in specificity vs sensitivity in gbm in R




I am trying to use gbm for a classification problem and my code to calculate the roc curve is:

pred.gbm <- predict(fit.gbm,[,1:18],
                    n.trees=best.iter, type="response")

pred.label <- 1*(pred.gbm>0.103) #1: >0.103; 0: otherwise
Confusion table

The confusion matrix is:

However the area is coming out to be around 0.50.I think this has to do with the probabilities to generate pred.label.
Is there a way to calculate a graph of specificity vs sensitivity to see the optimum cutoff in gbm??