You don't have to have the GPS-Cellular Watch for fall detection. However, there are requirements for the GPS version to make an SOS call.
Apple Watch Series 5 (GPS + Cellular) and Apple Watch Series 3 (GPS + Cellular) can use a cellular connection for Emergency SOS. To use Emergency SOS on an Apple Watch without cellular, your iPhone needs to be nearby. If your iPhone isn’t nearby, your Apple Watch needs to be connected to a known Wi-Fi network and you must set up Wi-Fi Calling.
ASC - Late 2012 Mac mini, macOS 10.14.6; Watch, series 5 GPS/cellular, watchOS 6.2.8; iPad Air 2, ipadOS 13.6.1; iPhone 8+, iOS 13.6.1; TV 4th Gen, tvOS 13.4.8; HomePods, audioOS 13.4.8 and Airport Express w/ Airplay 2