There were days when, at least temporarily, F1.5 was able to beat Red Bull – Riccardo got passed at least once by a F1.5 member. Being opportunists, there were times when Force India simply let Red Bulls through and other times when they… …didn’t. Ferraris and Mercedes were never knowingly impeded, even on the one occasion (Les Combes on lap 1 in Brazil) where it was just about possible that the advantage might have been kept had it been taken – because for either to do so would have required assistance from 3 other drivers, each from a different team, and this was unlikely to happen in a manner that suited both the FIF1s on either side. (Brazil, on the other hand, involved 1 driver from 1 team, and looked substantially more plausible). This is not to say Force India was 100% innocent of the charge; I just can’t think of an example that wasn’t explicable from the way in which the F1-F1.5 gap worked
In fairness to Gunther, I’ve never seen a Haas driver do any sort of differential driving of the type he’s complaining about – the times when they were impeding drivers, they were completely even-handed in how they did so. I’m not sure the same was necessarily true for, say, Williams (Singapore springs to mind – and not only was a Haas was trying to race the Williams for part of the time I have in mind, but also got penalised for ignoring blue flags while attempting to race said Williams. This would further explain why Gunther is so incensed by all this).