Dyer traveled to South Africa with the Dragons to participate in the Challenge Cup match against the Lions on Saturday in Johannesburg.
“He just keeps things going,” Tiatia said.
“He's back in the saddle very motivated and has been impressive this week.
“We are happy to have him with us but we are not happy that he was not selected. The more players get opportunities to play for their country, the more they will come.”
Three Dragons players have been named in Gatland's squad with number eight Aaron Wainwright and hooker Elliott Dee traveling to South Africa and scrum-half Rhodri Williams remaining at home recovering from a minor knee injury.
Captain Angus O'Brien is out with a thumb injury and Kai Evans faces a lengthy spell on the sidelines with a serious Achilles problem, leaving Will Reid and Lloyd Evans as the fly-half options in Johannesburg.
The Dragons need to avoid defeat to the Lions if they are to sneak into the knockout stages of the Challenge Cup.
A draw would see them through on points difference, with the Lions needing to try for a bonus point if the Lions cross the Dragons line four times.
A last-16 finish would be a welcome respite from the United Rugby Championship (URC) as the Dragons sit bottom of the table with eight defeats in nine matches.
