Setting GOP size on 'WMVideo9 Encoder MFT' encoder

I have to set the GOP size on 'WMVideo9 Encoder MFT' encoder, but I don't know how. I didn't find such attribute through the IMFAttributes. Also, I would have to tell encoder to use closed GOP. The output format should be MFVideoFormat_WVC1.

Regarding GOP size, the closest I got is MF_MT_MAX_KEYFRAME_SPACING. But, it doesn't look like it will set the GOP size.

Can somebody please comment this?

Should I call QueryInterface() on the IMFTransform in order to get some interface that provides a way for setting the GOP size?

Thank you!