I would be surprised to see Marshall do a GPS dog product. It doesn't make business sense.
Some years ago I talked with Wildlife Materials about plans to do GPS dog collars. By then Garmin was entering that market. They had no plans to do dog GPS based on the idea that any time they wanted to Garmin could slam all the other makers to the wall on price.

Secondly our access to Marshall grade falconry telemetry rides on the coat tails of the Persian Gulf market. Until HRH Shiek (pick one) decides he needs a couple hundred dog collars it's not going to happen.

Check up on TinyLoc if you need a GPS alternative to Garmin.

Thomas of the Disappointments

P.S.: I have a personal connection with a source of simple 433 and 434 MHz beep transmitters that are compatible with Marshall beep receivers. Note: You are on your own for the collar.