"He lost five kilograms."
Jin is a measure that's about half a kilo and probably similar to the pound (lb) in English. Think of gongjin as a long pound or kilo. Literally i guess it's the public weight measure.
My teacher told me that they tend to use jin for small amounts and gongjin for more than a couple.
In this example sentence, the 了 is used in a way that indicates a change of state. 他很瘦 = He is skinny. 他瘦了 = He became skinny.
There are also other ways that you can apply an adjective to a noun with intensifiers which mean that you don't separate the noun and adjective with 很. 例如：他瘦極了 (他瘦极了) = He is extremely skinny. (This is introduced in Food 3, by the way.)