I think the problem is the gtmedia receivers. Have tried 3 lnbs and no verticals, but get verticals with my 3 old receivers on all lnbs.180 degree rotation would not change polarities. The polarity will remain the same. Did you instead intend to say that the LNBF was rotated 90 degrees and vertical polarities were received?
It seems to be something specific to your GTMedia STBs. I don't have this brand, so unable to test and comment. Maybe another member has a GTMedia STB and could comment if they have experienced any anomalies.