Hi Xue,
This is a miscellaneous finding with a variety of possible differentials.
It still could be a GCT.
The GCT would arise from the tendon sheath itself rather than from the tendons within the sheath .
The tendons slide within a stationary sheath so if it was a GCT of the sheath I would expect to remain stationary just as this one does.
So it is behaving like something arising from the sheath.
It could also be arising from the palmar aponeurosis like a dupuytrens lesion.
It could also me a post traumatic lesion.
Many possibilities I think !
Let me know if you ever get a final diagnosis.
Steve