idk why people are making such a big fuss over the fact that the new GI connects with all hit levels. Rarely does a GI fail in SCIV because of an incorrect hit level, in my experience. Timing is almost always the deciding factor.
This is just the way movelists are in SCIV. In the SCIV GI system, timing must be predicted first and foremost, and then the hit level. However, it's only a mixup between a high or low in this situation since both 6G and 3G counter mids.
So the high and low have to be at the same timing in order for hit levels to be relevant. How often does that happen? Against most characters, the attack that's coming is probably a high, mid or throw. If there is a low that they'd use, it's probably not at the same timing as their high anyway! And against those characters, 3G is better since their highs are almost always weak if they have good mid/low! (Amy is an exception but her 6BB comes much earlier than her mid/low BnBs)
It's never a hard decision whether to 3G or 6G, so I say the change to covering all hit levels in SC5 is very very minor. The prediction will be in the timing, since highs and lows rarely come at the same time anyway.
Really the only time you have to choose between 3G and 6G is in the situations where 2K is a good move. When they have a sliver of health left and when 2K is the character's fastest non-high interrupt (Rock/Asta/Sieg/NM)