I got a BTO 5.1 that's still under AC. I replaced the BTO GPU, APPLE SSD with onther components. I it oke to sell them or should ai keep them to restore it back to original BTO in case I need to use AC?
As far as I understand AppleCare, that components are under warranty just as part of that machine.
For genuine Apple parts: for example if you bought Apple Radeon HD 5770 as upgrade kit, you cannot get AppleCare for it (3 years warranty), just 1 year limited warranty.
So, of course you can sell your GPU, but it is out of warranty outside original machine.
If it's the GPU and SSD only, you can sell them and don't worry. These are user serviceable parts. If it would be CPUs, I'd keep them. If you'd need to sent your MP for a warranty repair, make is as low end as possible. If they would have to replace the whole machine under AC, you'd lose all upgrades, because they won't care what's inside.
Years ago when I had a 2008 Mac Pro with AppleCare, I sold the GPU, Radeon 2600XT while AppleCare was still active at that time. Later upon claiming warranty to replace a hard drive, there was no problem and warranty was granted without Apple asking about the 2600XT.