I bought a GT avalanche 3 second hand with seized forks. When I stripped them down to clean, there is a coil spring in one side only and no damper fitted.
No. The fork doesn’t have a damper and never did.
No. You can’t retrofit a damper to these forks.
Possibly you might be better served with a pair of rigid forks or hit up your LBS to see what they have in the salvaged parts bins.
New 1-1/8 26” forks are available however that’s maybe not a good investment.