Flanker Turnbull is a seven-time cap for Wales and has played nearly every minute of every GUINNESS PRO12 game for the Blues so far this season.
They join nine other Blues players in the squad, and the Welsh region are seemingly on the rise under head coach Danny Wilson after three wins on the spin in the league.
"Tom (James) is playing very well for the Blues and we have been impressed with Josh's (Turnbull) performances recently," said Gatland.
"It is always exciting when the Six Nations comes around and we are looking forward to meeting up next Monday.
"We have picked a larger squad than normal, we have a few injuries at the moment, those players aren't too far away from getting back on the field so we are happy with the mix.
There is also space in the side for Aled Davies - who has caught the eye this season at Scarlets sharing game-time at scrum-half with an already established Wales star in Gareth Davies.
"Aled (Davies) is someone who has been on the radar the last couple of years and his selection is reward for his form," Gatland added.
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