Hmm... My understanding is that VSync should sync with the monitors refresh rate, not the room speed... but then if your setting the refresh rate to 144 and the room speed is 60 it will stutter. If you try adjusting the room speed to 144 to match the players monitor then that also won't work because it will then speed up your game by 2, which isn't what you want.
Ultimately, I think you will need to use delta time to achieve this, otherwise the solution would be for the user to set their refresh rate to 60 while playing the game. I don't know if you could possibly do that in code somehow, but I kind of doubt it. Unfortunately adding delta time will not be easy, especially if your already far along with your project. Delta time changes pretty much the way you calculate every variable, specifically variables that apply motion or that change over time. Things like particle systems, alarms, motion functions (like motion_add), speed related variables like image_speed, speed, friction and many many other things do not work with delta time and so you will need to rebuild them with your own custom systems. Every place you have an alarm set, or image speed, or object speed, etc all need to be rewritten entirely. Not to mention there are also things that just don't work with delta time at all, like physics or the built in particle system.
So, not trying to dissuade you, but it's important you understand how difficult a task like this will be. You will essentially be committing to rebuilding the game, from scratch. It may not be worth that if you are close to release, realistically how many users will be affected by this? Most all people are at 60fps.