The union and company officials will give evidence to MPs on the Transport Select Committee on Tuesday about the Southern chaos.
A GTR spokesman said: "We will be confirming details of a temporary timetable later this week.
"The timetable recognises that the present level of service has not been acceptable and will aim to give passengers a more predictable service they can plan around.
"This will be a temporary measure until train crew availability levels return to normal."
A Transport Department spokesman said: "We are clear that GTR passengers are not getting the service they deserve and the current situation is unacceptable.
"We are aware that GTR is considering some changes in order to strengthen their timetable in the face of ongoing disruption, so as many services as possible can run.
"When this happens, operators are contractually obliged to inform us in advance, but these decisions are taken only when there is no other solution, and this does not amount to the Government giving permission."