Yes there is a lot of gold left. Many people that look along rivers and creeks don't take into account where the rivers flowed 1,000 to 10,000 years ago (ie. Benches). Knowing your detector well will help you find things that people missed (proper manual ground balancing is very important - auto ground balance often helps you miss good targets - I use ground grab on my GMT and then adjust the ground balance manually from there on - you can improve your sensitivity greatly, if you know what your detector is telling you). I also use a smaller coil (very rocky where I detect). Most people don't have the patience to really detect/hunt effectively. Its easy to think there is nothing left, when you don't take the initiative to study the geology of a place. If detecting for gold was easy (its the hardest type of detecting I know of), gold would not be so valuable.
In the desert, you have a different set of rules, but people keep finding nuggets at Rich Hill Az (it's hit with some of the best gold detector gear on the planet, almost daily - but there is still gold found there).
If you are looking for gold ore that is bound in sulfides, your detector will be of little value, even when there is a lot of gold all around you. Learn what was found at a site in the past, and get to know what the finds looked like.
