Well, I have been following some good discussions about G haplogroup on Facebook and FTDNA. It seems that, in recent months, some progress has been made in conjectures concerning its evolution in time and space, thanks to a higher number of YDNA sequencing data, SNP testing etc. As reference, here is a very recent update from the expert R. Banks: sites.google.com/site/haplogroupgproject/-european-migrations
Update: Now he's suggesting that L497, which has the most G data now, is very European, and "also shows the major splitting from inside a small group also during the European Bronze Age". According to him, L497 may have come from Northern Caucasus with the Corded Ware culture or after it, both showing much "the same distribution today that was ascribed to the Celts when names were finally applied to the Bronze Age peoples". Still in his words, "the Celtic distribution and the Corded Ware people have heavy geographical overlaps", and L497 "did arrive on w. Mediterranean islands and western Scandinavia, but it is not clear if this was from the initial spread or secondary".