Thanks to everyone who participated so far.
The dating by our forum-colleagues is based on (supposed) internal references in GMark to historical events and circumstances (outhouse, Bernard Muller, Stephan Huller, Giuseppe, Charles Wilson, perseusomega9) or references in GMark to other texts as sources (Paul the Uncertain, mlinssen, Giuseppe) and external references in other texts (Ken Olson, JoeWallack).
First, it seems to me that Joe is right that Justin is the established terminus ante quem.
Why not Papias?
My problem with Papias is that we have no extant writings from him and - as long as I haven't missed anything - only Eusebius claimed that Papias already reported that Mark wrote his Gospel as the interpreter of Peter.
The tradition about Mark as the interpreter and companion of Peter is old (Old Latin prologues, Irenaeus of Lyons, Clement of Alexandria), but no one before Eusebius claims that Papias has already reported it and it is only Eusebius who claimed that Clement reported it in agreement with Papias („Clement in the eighth of the Outlines sets forth the record, and the Heirapolitan bishop, Papias by name, also testifies with him …“).
To me, the supposed account by Papias about the Gospel origins is therefore not sufficiently certain.