They won't get rid of your noise floor. They provide a "clean" reference ground for the signal which can eliminate alternator whine, pops, chirps, and other electrically induced noises. GLIs aren't really rated in amperage, as the signal current is extremely low. There's inline power
noise filters that are necessarily rated in amperage. Those are just capacitors to filter a/c ripple on a d/c line. Different thing altogether though.
My take on GLIs: while it's always best to have the fewest possible components in your signal path, the reality is that there's a ton going on electrically in a car. In most instances of noise where all they typical stuff (wires routed and terminated correctly, etc.) is addressed, they can be lifesavers and will not add any audible degradation of the signal. There's no reason to preemptively install one though.