Is zakah due on all the gold or just on that which exceeds the nisab?

You have to pay zakah on all the gold that you have. 

The nisab for gold is 85 grams. If a person owns one hundred grams, for example, he must pay zakah on the entire one hundred. 

This is indicated by the hadeeth narrated by Abu Dawood (1572) from ‘Ali (may Allah be pleased with him) who said: The Prophet (peace and blessings of Allah be upon him) said: “If it is two hundred dirhams, then five dirhams are due, and if it is more than that, it should be worked out on that basis.” Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

This indicates that zakah is due on the nisab and anything more than that. 

The amount to be paid is one-quarter of one-tenth (2.5%) which may be paid from the gold itself, or the current value may be worked out and this may be paid in the form of cash. 

And Allah knows best.