Owen Wengerd is right. Most probably function signature doesn't match the base class.
Check following 2 parameters:
const AcArray<Adesk::GsMarker>& gsSelectionMark
const AcArray<Adesk::GsMarker>& keyPointGsSelectionMark
If you use AcArray<int> instead of AcArray<Adesk::GsMarker> then it's OK on 32-bit machine and won't match on 64-bit. Adesk::GsMarker is IntPtr which is 64-bit size on 64-bit platform while int is 32-bit.