Nice job, @alex628! I've been taking the same route, building tiny little ggplot2 extensions as a way to learn package development.
AFAIK there are two places—unfortunately similarly named—where you usually need to signal that you need another package's function. One is in the DESCRIPTION file, where you already have Imports: grid.
The other is in the NAMESPACE file. I see that your offending line is inside GeomQuantileFrame, which is decorated with a # @export comments. If you also add # @importFrom gridTree, roxygen2 should add that function to the NAMESPACE. You also shouldn't need the package-qualified version of the call, grid::gTree, since you also have grid listed in your DESCRIPTION file.
EDIT: I might be leading you stray with the theory here. I'd recommend having a close look at the linked section above, 'cause it really helps to untangle the relationship between NAMESPACE and DESCRIPTION, but I'm def not an expert on this 