Monitor first before GPU.
Reason: Monitors have a longer "lifespan" than GPUs (which is updated by Nvidia/AMD at a faster rate).
Getting a very powerful GTX 1080Ti paired with a mediocre monitor (for example, 1080p 60Hz) is overkill and not worth it. This means you cannot take advantage of the capabilities of such GPU because the monitor is the one hindering the GPU max. possible performance. Furthermore, if you shell out most of the budget for an expensive GTX 1080Ti without a suitable monitor, it may take you a long time before you can get an equally-expensive monitor. During such waiting time, your GTX 1080Ti would have significantly decreased in value (not to mention, cannot maximize its potential), and a lot of better GPUs would have been released or would have been made available when the time comes you will be getting the monitor. As such, you'd probably be forced to "upgrade" again your GPU to a better one in the future (esp. if gaming requirments become higher).