[zzhk]

Sirzechs was talking about what would happen if Ophis gained control of the gap after Great Red was removed.
Great Red is not maintaining the gap, but his presence prevents others from taking control of it.

Since Ophis has spent so much time in the mortal realm, she might have been corrupted and may interfere with the other worlds' interdimensional travel.

Of course, by now we know the true nature of Ophis.