yes you can. With synchromesh, the gears will not actually slide together if the revs are wrong. there is no great advantage, it will be slower than a gearchange with the clutch. I suppose it will wear out the synchro cones faster. If you want to try it, you can push on the gearlever as you are slowing down, towards a lower gear, and you wil find it will not "go" until the revs are right.
It is commonly done on motorcycles for a fast change, which have a different kind of gearbox and very little inertia to the engine. If you get the revs wrong you get what a motoring pioneer described as "prolongued side-grubbing off the teeth"
You can't do it from a standstill, though since the wheels are not moving, and the engine is, so you can't match their speeds.