Hey Brendan
I don't know if anyone has replied to you but I'll throw in my two cents.
You can find Gervais Farm on Mason Libby Rd. I've parked in two different places, by the barn (which is across from Black Point Auto and Towing) and in a small parking area where Washington Ave meets Mason Libby Rd. As far as I know it is fine to park at either location.
From there you will see a number of trails which mainly head off towards the marsh. One "road" from the small parking area heads to the tower (I can't remember if you can climb it or not). The area is currently very icy, wet, and overgrown in some areas so dress appropriately. Really you would do well to explore the trails and get a sense of the area yourself. Use good judgement about the trails and where to walk.
As for looking for the shrike (there are others on this forum who will have much more and better information about looking for shrikes than I do) but I've had luck at Gervais Farm by scanning the tops of trees looking for the bird perched as it scans for prey.
I looked for the bird a couple days ago and was unsuccessful, but I was able to find the previously reported Northern Flicker and a Mourning Dove that I was convinced (from a long distance) was the shrike.
I hope that helps. Good luck.
Nathan Hall
Portland, ME