Not to be a party-pooper, but I have to agree with Guest Q. The connection of the Robin Hood of the ballads to the Green man is pretty tenuous. However, it is likely thathis adoption into the May Games is at least influenced by people's associations of him with green, summer, etc. His appearance, for example, in Hal An Tow and other may songs clearly links him with summer, vegetation, etc. So the association was there, and the question is just how conscious it was in the minds of 15th and 16th century people.

The same goes for the Green Knight, but of course at an earlier time. Sir Gawain and the Green Knight obviously borrows from seasonal mythology, you can't get around it. But those borrowings could signify a connection in people's minds between the Green Knight and pagan gods, or they could simply have come down as narrative conventions from earlier stories that had seasonal meanings. Curiously, if we go with the seasonal interpretation, the Green Knight (although green) is in fact a winter figure, like Arawn Pen Annwfn in Welsh mythology. The Green Man, on the other hand, is a summer figure.

My own suspicion is that the Green Knight was more a matter of narrative convention than ancient mythic meanings, and I suspect the oral tradition of folktales provided the link between older myths and medieval romances. Stories like "The King of Ireland's Son and the King of Green Island" contain many motifs in common with Sir Gawain and the Green Knight, and may have provided all the necessary narrative material for the romance without much serious consciousness of the seasonal mythology. On the other hand, SGGK is explicitly a seasonal poem, set on New Year's Day of one year and the period leading up to New Year's Day on the next, so it's pretty obvious that SOME consciousness of the seasonal meanings was operating. It's all a judgement call!