In Storyteller, Gwernin's travels take him in a circuit around Wales, beginning at Pengwern and ending at Llys-tyn-wynnan. We first meet him at Caerleon, admiring the Roman ruins there. (This map was redrawn for the e-book edition.)
Flight of the Hawk
In Flight of the Hawk, his journey is longer, but still a circuit; he starts at Llys-tyn-wynnan and eventually returns there, passing through Deva, Elmet, Rheged, Goddeu, Aeron, Strathclyde, Manau, Pictland, Gododdin, Bernicia, and Deira, among other places. (Note: the dotted lines on this map represent old Roman roads, not Gwernin's itinery.)
The Ash Spear
Finally, most of The Ash Spear takes place in Gwynedd and Powys, although Gwernin also visits Elmet in the latter part of the book.
Gwernin's next trip is to Ireland - but that's a map for another day!