I've never thought of them as 'Celtic', but forced into a corner, I'd probably say Gallicia was Celtic, it does have echos of Breton and Welsh, but not so much Asturias, which comes across as more Spanish. But this whole label of 'Celtic' does not sit easy for me. All of the regions and countries under this label have their own distinctive music.
As to the Basque, they seem to have more in common with English traditions, like hobby horses, morris dance, etc. On a recent visit I saw schoolchildren being taught morris stepping very like the English version.
But I expect to be shouted at for saying this!