Hi there,
Just like 1990LS400 I do my own oil changes on my 2006 GS300 SEL and I do not need to use ramps.
I have ramps but I hate using them as the risk to the car is high when trying to put the car onto the ramps, I once ran an autin mini completely over the end of the ramps trying to ram the car up onto them since I was chasing the ramps down the road (they were sliding) !
I change my oil on the drive and the sump plug can be reached by lying on the ground with my right shoulder just behind the RHS front wheel. The socket wrench fits nicely on the sump plug and it is fairly easy to manipulate by feel.
The rest of the oil change can be carried out from the front of the car but remember that you do need the special tool to remove the oil filter housing. You may also need to remove the cover underneath the front of the engine compartment directly under the filter housing to get some leverage on the filter housing but this can easily be reached from under the front of the car, you will need a 10mm socket and wrench for the cover screws.
I hope this helps ...