There are two branches of Celtic: P-Celtic and Q-Celtic. Q-Celtic includes the language of the ancient inhabitants of Ireland and those speaking a language derived from it--Irish Gaelic, Scottish Gaelic, and the recently-extinct Manx. P-Celtic is everything else, including the language of ancient Gaul and the language of ancient England and the languages descended from it (Welsh and Breton). Some of the placenames in southern Scotland are P-Celtic so a language of the P-Celtic variety must have been spoken there before Gaelic was introduced from Ireland.
I don't know how Pictish fits into the picture. Apparently there are some very old place names in the British Isles which are Indo-European but not Celtic. Of course there were people in the British Isles long before the Celtic language reached them--the people who built Stonehenge did not speak Celtic.
He dressed like a Jain of the time and taught what can only be Ahemsa. Gildas means "Happy Servant" in Sanskrit.
The peculiarities of the Picts may extend no further than having a different history ~ a really different history ~