Gw- as a verb prefix particle? Parallels to German?
It occurs to me that there are several Welsh verbs that begin with the letters gw-, like gweld, gwneud etc...
Could this be a verb particle roughly parallel to ge- from several Germanic languages, like German, Dutch or Old English? The particle usually indicates completeness, like gesprekt means something that has been spoken or gewyrcan is something that has been worked or made.
I have completed both the Welsh and Irish trees and both languages seem less similar to the other Indo-European language families I have studied (Romantic, Germanic, Slavic and a little Baltic) than they are from one another. The prospect of finding such a parallel is interesting to me. I have always assumed that the older versions of the Celtic languages had more parallels to the other, older IE languages that have been lost over time, like parallel systems of noun declensions and verb conjugations.
No; it's not a prefix.
Old v- or w- often turned into gw- in Welsh; compare gwin with English "wine" or Latin "vinum", and gweld is apparently related to Latin "vultus" (appearance, face), and gwneud to English "work".
It is not a verb prefix, really, as many nouns etc also start with gw-
If you look in Geiriadur Prifysgol Cymru (GPC), the authoritative Welsh to English historical and etymological dictionary, you wil often find notes on word origins and related words in other languages. The GPC is available on-line and as an app.