Gatland, who turns 23 tomorrow, has signed with the Highlanders for the next two seasons and will come to the franchise next year to add to the depth in the No 10 jersey.
Gatland has been with the Blues this season but will head south as his path is blocked in Auckland by Stephen Perofeta and Otere Black.
Lima Sopoaga is leaving the Highlanders at the end of the year to play in England.
Highlanders chief executive Roger Clark said the Highlanders now had Josh Ioane and Gatland on their books as first five-eighths and were still looking round for an experienced No 10.
Fletcher Smith, who can play pivot as well as other positions in the backline, appears to be on the outer.
Clark admitted the type of experienced first five-eighth they want does not grow on trees.
''We have a talented team which is capable of winning championships so we need an experienced guy who can steer the team round the paddock,'' he said.
''He can teach the young guys and bring them along.''
The franchise had until the end of October to announce the player and Clark said they were working through the process.
Also announced yesterday was the signing of loose forward Marino Mikaele Tu'u.
He was called into the squad as an injury replacement for James Lentjes, and has impressed the coaching staff with his power and durability.
The Hawke's Bay product, who played for the New Zealand Colts last year, will stay with the franchise for the rest of the season and has signed for next year.
Winger Tevita Nabura has also signed as a fully contracted player for 2019.
The hard-running Fijian winger impressed when playing one game for the Highlanders.
He has also played well for the Highlanders development side, the Bravehearts, and his Zingari-Richmond club, Clark said.
The franchise announced the signing of All Blacks Waisake Naholo and Liam Squire for next season, along with others, on Monday.