You do not need a license to purchase and use the detector. But in some countries, you must obtain a license from the local authorities to dig.

Pure gold alone cannot stick to a magnet. However, if you have a gold ingot it can stick to a magnet. An example of a gold alloy that may stick to a magnet is gold in which more than 20% of its atoms have fused with an iron.

Modern prospecting detectors such as the Klayzer GR Pro can detect gold as small as half a grain.

The best places to detect gold nuggets are places where gold was previously found and surrounding areas with similar geology. So again, do your research to learn about the historic gold fields. Ex: tailings from gold mine sites.