Not sure what GR20 is but I do know a fair bit about knee pain, especially when descending when running or hiking or even going down stairs. Provided you do not have any underlying knee injury issues the most common cause I run across for runners who get knee pain is tight iliotibial bands. The IT band is a long tendon that runs the length to the outside of your thigh and and connects a small muscle up near the iliac crest of your pelvis to the front of your tibia (shin) just below the knee cap. When this thing gets super tight it causes pain that will seem just like a bad knee. An easy fix for this is to roll the IT band regularly. Have you read this: https://uphillathlete.com/practical-recovery-essentials/. If you can not bear the pain of putting full body weight onto a foam roller on your IT band then there is a good chance you have found your problem. Look on Youtube for videos on IT band rolling. Rolling will beat stretching here. If it hurts like crazy to rill on it then your job is simple roll that sucker several times a day until you can put full weight on it. Stop with the back squats and start doing single leg step downs as shown here: https://uphillathlete.com/strength-demo-box-step-up-and-heel-touch/. If you’ve been doing lost os heavy squats (like 5×5) then chances are good that your issue is not strength in the prime movers but strength in the stabilizers like glute medius.

Good luck,
Scott