For the purpose of recording your trails for Top Trail, you are free to use any GPS device designed for outdoor/recreational use or a GPS app installed on your smartphone. There are many to choose from: some apps free, others are not but they are relatively inexpensive.
When choosing a GPS, the critical feature/requirement necessary to work with Top Trail is the ability to export tracks (your recorded trail) from the app or device in .GPX file format. Top Trail requires a .GPX file to calculate your mileage and render your thumbnail trail image.