How do you get free gilded armor in Runescape

There is no such thing as getting free gilded armour or any variation of "free expensive stuff please"; unless you're doing Treasure Trails, it's highly unlikely you'll be picking up valuables from the street that someone happened to just drop because they felt generous. No, people tend to sell their unwanted stuff through the grand exchange rather than carelessly tossing things aside. ;)

Asking about freebies is like saying "I'll drop my 200m bank right here for anyone to take", which is a very, very, very low chance of any sane person doing. ^^; So I'm afraid you're out of luck unless you actually take risks -- such as bossing -- or finding clue scrolls to begin a treasure trail and reap possible yet random rewards for your own profit.