Has anyone found a history for Angel's gryphon tattoo in script notes or source books? We see it a few times on both shows, but they never talked about why, when, where he got it. I'm just really curious if there's something I've missed.

Don't know about the canon of his tattoo but I read online that Joss Whedon wanted a tattoo on Angel - in mythic symbols the placing of tattoos on the body is a symbol of the magic and spirit being manifested in the real world in a shaman or representative of that magical entity.  Another interesting symbolic connection to Angel, aside from the fallen man and two faces of humankind and life forms of earth, is the Catholic "ringing of The Angelus" the magico/spiritual energy connection between God and Man being sent to the earthly plain.  The angelus - ringing of the angelus is the call for prayer in the Catholic tradition.

At first the Angelus consisted only of the first part of the Hail Mary, repeated three times. This was prescribed for the success of the Crusades and the recovery of the Holy Sepulchre.

The Gospel narrative which is summarized so beautifully in this devotion is found in the first chapter of St. Luke, from which two of the versicles and responses are taken, the third being from the Gospel of St. John. Thus, by reciting it, we are reminded at morning, noon and night of Him Whose Name is "the only one under heaven given to men whereby they may be saved," and of her who is well entitled " our life, our sweetness and our hope."

This, IMVHO, makes a beautiful connection to Angel and Buffy, and how Buffy works as a symbol of potential salvation, not only to Angel but also to Spike.

Isn't it a Book of Kells inspired tattoo?  It would make sense, what with the Irish connection.  As for a canon explanation for it - no idea.  I don't remember one on either show, but that could mean I just missed it.