The really simple way to do it is to invert the gloss map and use this for Roughness - you may need to do a bit of an adjustment on the levels to get it to look perfect. Keep in mind that a rough surface (like rock or bricks) will have more white in it and a smooth surface like glass or chrome will be completely black or close to it.
The specular map usually has the metallic highlights in it (scratches and nicks) so you can also Add this to the gloss map before you invert it if you want to get those extra details in there. It depends on how well the specular was created, but overlaying them works pretty well to get the right blend. Any area that is scratched in a specular map should be highlighted in white, but in a roughness map this would cause them to not shine, so you want them to be black hence overlay and then invert. It kind of depends on whether you want the nicks and scratches to catch the light, or be dull. If you want them to be dull, then you would want to invert the spec before overlaying.
Then for metallic, anything that is metal should be pure white, anything non-metal is black. It’s really that simple. A lot of people will use the metallic to try to tweak the look of their roughness map by using a greyscale texture, but you shouldn’t do it that way as it is physically incorrect. A metallic or metalness map should always be black and white as there aren’t any materials in the real world that are partially metal. If it doesn’t look shiny enough, then you adjust your roughness map to be darker in those areas and it will, in turn, make the metal shine more and vice versa.