Quick answer: True real-time GPS trackers without subscriptions are rare — most need cellular data to work. The best no-subscription options use Bluetooth community networks like Apple AirTag.
Real-time GPS tracking sends location data via mobile networks to your phone. That cellular data costs money, passed on as a monthly fee of $5-15. Without a plan the tracker can detect GPS coordinates but can't send them anywhere.
Apple AirTag ($29 one-time) uses the Find My network — detected by nearby iPhones that anonymously report location. Works well in cities where iPhones are dense. Samsung SmartTag uses the Galaxy network similarly. Neither gives real-time tracking but both show last known location reliably in urban areas.
Some trackers offer a free tier with a few location updates per day and paid upgrades for real-time tracking. These are worth considering for indoor-outdoor cats that don't roam far — the free tier is often sufficient for finding a missing cat within a few hours.
For urban cats, an AirTag is hard to beat for cost. For a cat that roams fields or rural areas, cellular GPS with a subscription is more reliable despite the ongoing cost.