check the information center http://www.capcan.ca/gatineau/useful_info/how_to_get_here_e.asp#year_round
to get a map of Gatineau park and how to get to Phillips Lake. I usually walked to the right away from the beach about 100 meters and cast there. You can find plate rock there and can't miss the spot. Use worm or plastic bait for large mouth Bass. Autumn should be good season again to catch the big fish.
Actually you can find more nice places in north area for fishing.