First of all, welcome to the better side of gaming (GO PC GO PC!).
I'll do my best to make this as straightforward as possible.
"Tearing" in PC games occurs when your GPU is putting out frames faster OR slower than your monitor's refresh rate. If you have a 144hz monitor, that means your monitor is refreshing the image on the screen 144 times per second. If your GPU is only putting out 100FPS, that means your monitor is refreshing nearly 1.5 times for every frame sent form the GPU. This causes the image to tear because they are not synced so you will see a portion of one frame sent form the GPU and a portion of another frame from the GPU on the screen at the same time. This same phenomenon occurs when you have a slower monitor refresh rate and your GPU is sending frames faster than the monitor refreshes - just backwards.
No GPU will ever output the same framerate throughout an entire game session because as you move in game or more things happen in the game, your FPS will rise and fall based on how hard or easy it is to render what is happening in the game. Meanwhile, your monitor refresh rate stays constant.
What GSYNC does is this: It gives the GPU control over when your monitor refreshes so your monitors refresh rate becomes dynamic, as opposed to static. As your GPU produces a frame, it is sent to your Monitor. When the frame is received, the monitor refreshes the on screen image. So, regardless of how fast or slow (as long as it is above 30 fps) your GPU is able to render images, the product on the screen is buttery smooth with no tearing because 1 Frame Rendered = 1 Image Refresh on the monitor. Simply, GSYNC doesn't allow an overlap between the monitor's refresh rate and the frames sent from the GPU because the monitor only refreshes as it receives a frame from the GPU.
So to answer your question, YES there is a difference between a 144hz monitor with and without GSync. If your monitor does not have GSync and your GPU is unable to produce 144FPS, you will notice tearing. While the tearing won't be as pronounced on a 144hz monitor as it would be on a 60hz monitor because the image stays on the screen for a shorter amount of time, it will still be there.
I hope this helps to clear things up for you!