What is the difference between G-spaced & F-spaced pickups?

The string spacing between each pair of neighbor strings is different on different types of guitars. G-spaced pickup has a 50mm span between the low and high E string poles and is usually for Gibson guitars, and F-spaced has a slightly wider 53mm (or wider) span and is usually for Fender guitars or those using Floyd Rose Tremolos.

Why two different standards?

Originally, Gibson and Fender split the electric guitar market as two of the biggest suppliers. Gibson typically chose a narrower string spacing at the bridge than Fender, so the polepieces on Gibson humbuckers would be closer together than the magnets on Fender pickups.

How do I choose from G-spaced & F-spaced pickups?

Most of guitarists believe that the G and F-shaped pickups only have a visual or aesthetic effect. They wouldn't make a huge difference to the overall performance, tone, and feel. What you should do to choose a pickup is to measure the distance between high and low “E” strings and go with the closest match to make sure the low and high E strings are properly placed above the middle of the side poles.