Did you try modifying the the "Value" slider? This should modify the strength with which the glossiness texture influences your material. That said, lower value means less glossy, but what you'd like (if I understand it correctly) would be lowering the influence of the texture while keeping the material almost completely glossy. With the Metal material, you can do this by modifying the colors in the gradient or you can use a filter shader as nkilar said.
But if there's a lot of people who think this might be handy with some additional slider, we'll consider it. Just bear in mind that we try to keep the UI in Corona as simple and orthogonal as possible.
Edit: You can actually use the mix shader wherever you'd like to have a color and a texture and mix them together with the given mode and strength. So you can easily use it here for your purpose!
Regarding the roughness bar: roughness is actually the same as glossiness, because it can be expressed as inverse of glossiness (with some additional exponentiation, but that's not important right now), so if you want black roughness, use white glosiness instead and vice versa...
