Incidentally, as I was looking to expand this answer, you cannot get comments from a Stream or Commit query; to get comments from a specific commit - you can only return the number of comments.
Instead, you can apply a filter by resource to the comments query above.
If you refer back to the forum answer on attaching files API post comments with attached files - #7 by jonathon, comments are attached to a Stream resource; in this case, a Commit in this way, you can get just the comments filtered to this resource (both resourceType and Id are required):
{
comments(
streamId: "YOUR_STREAM_ID"
resources: [{resourceType: commit, resourceId: "YOUR_COMMIT_ID"}]
) {
items {
text {
attachments {
id
fileName
}
}
}
}
}