It is not:
> xgb_info <- caret::getModelInfo("xgbTree")
> xgb_info$xgbTree$parameters
parameter class label
1 nrounds numeric # Boosting Iterations
2 max_depth numeric Max Tree Depth
3 eta numeric Shrinkage
4 gamma numeric Minimum Loss Reduction
5 colsample_bytree numeric Subsample Ratio of Columns
6 min_child_weight numeric Minimum Sum of Instance Weight
7 subsample numeric Subsample Percentage
You can pass it in via the ... but it does not tune over it.