That is correct that I am not an expert in grappling rules, but I thought we did allow grapple attempts as AoOs.
On page 141 of the PHB, disarm, grapple, and trip actions have the superscript 7. In the notes under 7, it states that those actions can be used as attacks of opportunity.
It seems that the rules state the opposite of what Infiniti does: a creature with Improved Grab cannot use Improved Grab to grapple in an AoO because it is a free action, whereas a normal grapple attempt is allowed as an AoO.
I would allow both as an AoO in my campaign, and I am happy that Gansk does, as well, because I was once able to keep a bad guy from running away by using Improved Grab.
I think that Gansk should be allowed to keep the grapple AoO.
Maybe one cannot maintain the grapple because that requires movement, but the grappled creature uses up an entire move action when it is grappled.