So after thinking about it, my solution would be something like this:
iex(1)> tree = :gb_tree.example() #Any f that returns a tree
{3, {:a, 1, nil, {:b, 2, nil, {:c, 3, nil, nil}}}}
iex(2)> n = :rand.uniform(:gb_trees.size(tree))
2
iex(11)> iter = :gb_trees.iterator(tree)
[{:a, 1, nil, {:b, 2, nil, {:c, 3, nil, nil}}}]
iex(14)> {k,v,_}=Enum.reduce(1..n, {:nil,:nil,:gb_trees.iterator(tree)}, fn _, {_,_,i} -> :gb_trees.next(i) end)
{:b, 2, [{:c, 3, nil, nil}]}
However I am thinking, now gb_trees are not used any longer as we do have maps.
