ok if when your playing hl2 and you run across or see the gman there is usually something usefull in that area or your gona get something. also. ive seen in other forums that gman and gordon look somewhat alike and that ive came up with this explination.
if they look the same and gman seemingly keeps helping gordon why cant the gman be gordon of the future and for some reason (possibly due to aliens once again threatening earth AGAIN.) gman travels thru time at certain spots in the game to give gordon a little help?? i think that if hl3 were to come out that this would be the end story.
but also to the when u see the gman u get goodies. look around for the lambda logo usually on boxes that have goodies as well.