Gunther lapped the 2.4km circuit in the Indonesia capital in 1m08.946s towards the end of the 30-minute session, edging out his team-mate Edoardo Mortara by 0.191s to claim the top spot.
The two Maserati MSG cars were quick from the onset, with Gunther setting the early pace on a 1m10.811s and Mortara backing up his team-mate in second.
A number of drivers then enjoyed stints at the top of the times as the track rubbered in, including DS Penske pair Stoffel Vandoorne and Jean-Eric Vergne, before Pascal Wehrlein broke the 1m10s barrier in his Porsche to put himself ahead of the competition.
The Maserati duo responded with Mortara setting a 1m0.538s flyer before Gunther moved even further clear of the opposition with a 1m09.263s.
Source: Autosport