Have a low quality GPU in my PC. Can I fill the gap with an Xbox?

impossible, the xbox one x uses an apu so it is not a dedicated gpu that you can rip out and use.

there is no physical way of using the 1x as an external gpu.

I don't know if my fellow xbox'ers noticed or not that Respawn had an interview with Digital Foundry and they confirmed that the xbox gpu is based of Bonaire which is not the newest gpu technology. So a lot of misterx's stuff is conspiracy at it's best.
DX12 will help with optimizations but, it's not going to magically fill a hardware performance gap between the 2 consoles.

I don't think OP was trying to present this as what to expect in the console versions of these GPU's but an approximation on the performance gap. We all know an I5 Sandy Bridge used for that test system would beat that low powered 6 core Jaguar that is going
in the XB1 and PS4 embarrassingly in a PC. I think his point was completely missed.

DICE removed them because they were filmed and shown in low quality. The graphics themself are not low quality. Yes they will be "low quality" compared to the PC version, but they will still be pretty good on the xbox. Up until now, have people
who played it at E3 said that the graphics were bad? Not at all.

What's the solution?

Nothing, your GPU is not supported by Forza Horizon 3. You can set low settings from the initial configuration panel, but I strongly doubt it works.

Alternatively, try this:

- Go to "Options" in tha main menu;

- Select "Screen Resolution" from "Video" section;

- Set "Low Setting" from screen resolution;

- Set "Low" from "Dynamic Render Quality".

If it does not work, I'm sorry but I think you'll need to upgrade your GPU.