"Bjarni has shown a lot of commitment to Plymouth already and he has not even pulled a strip on," Williamson told the clubs official website.
"Just by the fact that he has decided to join us at this time of year, where he could have sat and had his Christmas with his family and got that out of the way.
"The boy's shown an appetite and the right attitude that he wants to play football, and that's great. I'm delighted to have him on board."
Gudjonsson could go straight into the Argyle squad for the game against Derby County this weekend.