1) A LOCAL WORK CONTRACT
This doesn’t need to be full-time but it does need to be formal.
Currently, you need a minimum of a 20-hour contract, or a contract for a defined period of work (as a consultant, programmer, etc), to get an NIE
If you are thinking about starting a business or start freelancing from Gran Canaria, you might be asked to write a Memorandum explaining your project, almost like a business plan.
2) INCOME, SAVINGS, AND MEDICAL INSURANCE
Spanish law states that you are eligible for an NIE number if you are employed in Spain, are buying a property here, or have a minimum income level that allows you to live here and not be a burden on the Spanish state and its social security system.
The minimum income is defined by Spanish law, but the police are also obliged by the law to take your circumstances into account; you need to have enough income to pay your rent, bills and living expenses before the police will register you and give you a certificate.
As a guideline, you need a regular income of at least 600 euros per month (per person for families).
Having significant savings in your bank account (at least 5000 euros also helps.
If you have an income but don’t work, the authorities will also ask for a certificate proving that you have private medical insurance equivalent to the cover offered by the Spanish state healthcare system. This costs around 1000 euros per year.