The following points give a brief overview of how search works on Gumtree.
Which fields am I searching?
Currently you are searching the ad title, description and location when you use the search box.
Is this AND, OR, or what?
Currently, the only option is a boolean AND. So, if you search for "red car" in the box, you will find postings with both the words "red" AND "car" in them.
How do I search within a category? Or the whole site?
The search is carried out in the category you are in, and all its sub-categories. So if you want to search the whole of a city, you can - just use the box on the homepage and choose "all categories". If you'd like to be more specific, browse to your chosen category and carry out your search at that level. This way you might get better results.