Based upon what I recall regarding the Digest Group Publications material for MegaTraveller, it's more likely that Marc Miller made agreements that transferred the copyright over to him, either before or after the dissolution of
GDW. The time frame is unknown to me, but given that the DGP material for MegaTraveller is held for ransom (the owner wants six figures for it) and doesn't appear as far as I know in the MegaTraveller CD-ROM that Marc produces, the voluntary transferral of copyrights to Marc is a far more likely scenario than anything underhanded.
For the record, when Starter Traveller was released, Marc's name appeared in it as the author, with the other three members of
GDW as contributors. Artwork was likely purchased under an agreement that transferred ownership and all republishing rights to Marc. As for the rest of the works, the rights may have been purchased for reasonable sums of money, or agreements made that transferred ownership. I wish I could say for sure I was right, but this is all speculative and I just don't know one way or another.