As you can see from the image below, the unstable wormhole vortex (known colloquially as the "kawoosh") comes out from the centre of the puddle, then retracts back into the centre. The ramp is below the area affected.
The ramp isn't a single piece, it has a small split in the middle through which the wormhole (and iris) can extend.
It was a production oversight in the earliest episodes that the ramp went through the gate. In later episodes, you can see the split more clearly.
You can also see in this original concept art, that even offworld (typically stone) gate platforms are built with a small gap to allow the vortex to form