You pretty much have it. Yes, a new layer. Put a ramp effect on it--start with linear, white to transparent. Then put a mask on the layer. Feather the mask as desired. Adjust the direction and balance of the ramp. Adjust the opacity of the layer.
Your method is unorthodox and may not give you the best result. I gave you a little misdirection, thinking that a ramp could have transparency in it. You correctly made the ramp white-to-black. You then created the transparency with a color key to remove the black. The effect is the unorthodox part. I suspect you are getting some gray superimposed over your underlying layer (over on the more transparent side) instead of a transparent white.
In AE, it's good to get familiar with blending modes. In this case, the reflection layer should use the Screen mode. That will get you the correct result. That is, only the white in the ramp will be superimposed. Try it. Just uncheck the linear color key effect and change the layer mode from Normal to Screen.
The feather on the mask is just an aesthetic choice. If you want a soft edge along that curve, you could feather the mask.
"Unless there's a plugin, I'd apply a white-black radial ramp to a layer then mask it with a curved shape. This I'd use over the original graphic in screen or add mode."
"To a layer" means "to a new solid" in this case. Applying effects to solids is a very common technique in AE. Example effects are lens flare (to a black solid, add or screen mode), fractal noise and ramp. In general, a "generate" effect is often applied to a new solid.