Yep, you need GIMX in order to run Forza 7 using your OSW.
The first step is to buy this adapter. It is created by the developer of GIMX and will work perfectly with it.
https://blog.gimx.fr/shop/
Both ends of the adapter needs to be plugged into USB ports on your PC. Install the latest version of GIMX, which you can get here:
https://forum.gimx.fr/viewtopic.php?f=2&t=2209
Once you install the GIMX software, you load up the GIMX launcher and you need to update the firmware to the G27/PC, which essentially causes the game to think you are using a G27, which is fully supported by Forza 7. The name of the firmware is EMUG27PC. Follow the instructions to update the firmware.
Once you have the correct firmware, go into GIMX Config, pick G27 profile and configure the wheel, pedals, shifters, etc. You can even use your keyboard for some of in game commands.
Let me know if you have any questions. I am helping test new versions of GIMX and so I have had a lot of practice doing everything.