The Galatians could be entirely naked in basic as well, since they are sometimes depicted like this in terracotta.
I would disagree with the Berru helmet since it is something found during the 5th century and the first half of the 4th century, while the Galatian migration started really in the beginning of the 3rd century. Probably that the most common Celtic helmets would have been the Eastern variant of the Celtic Montefortino series:
Finally it is also possible that Galatian elites used more Hellenistic equipment:
https://battleshields.tumblr.com/post/12433075574/galatian-shields-these-galatian-warriors-are
http://profesorjuliodapenalosada.blogspot.com/2015/12/le-soldat-lagide.html
The Boé, Port, Ciumesti and Coolus helmets are interesting but they are in use after the fall of Carthage (149 BC).
I think that it is possible to simply use different Celtic montefortino helmets, from simpler version to more decorated ones.