Blues boss Steve Bruce revealed earlier this month that he was willing to part with the Danish international who he said had just not settled at the club since his £2.2million move from Chelsea back in July.
This prompted interest from two or three sides from Spain and Atletico have secured the services of Gronkjaer once the transfer window opens at the beginning of January.
A fee has been agreed and Blues have confirmed and it is now up to the two parties to finalise personal terms.