As far as the specs go you got what you paid for however I don't think it was unreasonable for you to expect GbE in that kind of spec machine - likewise you won't expect USB 1.0 ports, 1x CD-ROM etc.
Try your luck with an RMA as not fit for purpose; they may well oblige otherwise count it as a learning experience to read the full spec and go with a USB 3.0 GbE adapter or wireless AC.