The last versions of the Shephard being influenced by the gospels wouldn't be that unexpectedI considered that option for a long time, but a closer look at the parable of the vineyard has convinced me otherwise. The version of the parable in the Shepherd is manifestly a mishmash of parables from the canonical gospels, especially Matthew. I mean, this one is not even a close call; it is obvious once the parallels are consulted. It is certainly possible, even probable, that the versions of the texts the Shepherd drew upon are not quite what we have in hand today, but the Shepherd drew on something like the gospel of Matthew, or at least on something which contained a number of Matthew's parables written with Matthew's now distinct wording.
Obviously, one might propose that the parable of the vineyard is a late entry into the text of the Shepherd and that earlier layers lacked this kind of connection with the gospels. But the extant version of the Shepherd depends upon something similar to the Gospel of Matthew
Instead of making the question - does the Shephard predate the gospels a better question might be - does it present an earlier form of Roman Christianity that that of the (proto) orthodox church later?
I think it's easier to say that it does and the layers in the Shephard account for the gospel connections plus the distinct possibility of non-canonical writings and parable collections influencing it too throughout it's history
The significance of dating the Shephard earlier is the doctrine found in it is one of the early Christianities, that based in Rome - presumably a focal point for evangelization - which shows many differences to the universal doctrine of the mid 2nd century
I'm sure it represents only one stream but it's a big one
So what i'm suggesting is it's a overwhelmingly vital and essential text witness to Christianity's development - it's uniqueness can't be blunted by assimilating to known later beliefs but these instead be emphasised and accounted for individually. That's how I see it. I'm unaware of the latest scholarship that might well have addressed all this and proposed good answers!