I don't think it would be very hard to defend most of what has been said about AJ here. A valid defence to an action in defamation is justification. i.e. the defamatory imputations were true in substance. AJ has put out more than enough content to defend most of these claims about his character with ease.
Defences aside, i think it's relatively unlikely that GAGR would be found liable for posts made by another individual - though the law says that it's possible. The issue lies in determining whether GAGR is to be considered a 'passive facilitator' or an 'active publisher'. A forum probably isn't an active publisher unless a potentially defamatory post has been brought to the attention of the operators of the site, considered and left published.
So, it's technically possible but a Judge could just as easily determine GAGR is a passive facilitator. Fairly shaky grounds on which to commence what would probably be fairly costly proceedings, i think.
Further reading about legal liability for online intermediaries for those interested:
http://classic.austlii.edu.au/au/journals/SydLawRw/2018/19.html
(not legal advice)