On the rail-way station Baltijsky in St.Petersburg get the electric train just to Gatchina (Gatchina Baltijsky station). It will takes no more then 10-15 min. from the station to the Gatchina's Palace (see the map below) and go ahead...
2. Parade-ground and the monument of Paul I
3. R/W Station Gatchina - Baltic
4. Gatchina's park.