But when I try to visualize my commit history using tools like gitk, it looks more like a tree than a graph since every parent-child relationship is directed one way.
A DAG, like a tree, can be laid out such that all parent-child relationships are one-way. The difference between them is that nodes in a DAG can have multiple parents. The most common case of this in Git is when you do a merge. A merge commit will have all of the commits that were merged as parents. A tree doesn’t allow nodes to have multiple parents.
(Image source)
Notice how the merge commit C6 has two parents, C4 and C5.