I just did that fix for the exact same reason. I bought a cheap Mavic Pro in mint condition without a scratch on it with a gimbal that didn't work. The ribbon cable on the gimbal was ripped. I bought a new ribbon cable from Amazon for $11.98.
Amazon.com: Rantow Flexible Gimbal Flat PCB Ribbon Cable for DJI Mavic Pro Gimbal Camera PCB Repair Cable: Toys & Games
There are numerous videos on youtube explaining how to replace it. I will tell you this, search for these videos yourself and watch as many as you can until you feel comfortable. I will not tell you which videos I watched because many of them do not give you good views or angles and in my opinion and are hard to follow. It is impossible to do it from one video. I had to refer to at least 6 of them and it is very time consuming if you've never done this type of repair before. I do not know what your technical level or skill is, but it's not impossible to do and you will need to buy some specialized tools to do it or you will most definitely find yourself very frustrated. Small tool kits similar to what cell phone repair guys use. Small screwdrivers, torx and allens. Plastic spreaders, tweezers and the like. Even exacto knives because there is glue you will need to cut away. You will see these tools in the videos. That being said, my gimbal is now working, but not perfectly. I have complete control of all functions, can fly just fine and control the camera in every aspect, but the camera is looking to the right every time I power it up to fly and my gimbal calibration fails every time. Not sure if that is because of something I did during the repair or another glitch of the Mavic Pro. Seems a lot of people are having my issue and I am still trying to figure it out. Every time I bring it up I just have to turn the camera looking straight after it comes up looking to the right and I can fly. It's just frustrating that it never comes up with the camera looking straight. See my pic below. I already had the tools, so total repair cost me the cost of the ribbon cable. Cheap!
Good luck!
Paul aka Mavicpro66