Can you do math in your head? Memorize a gear chart? Mount a stopwatch on your 'bar. Count pedal revolutions. If you know pedal rpm and gearing you can calculate speed. Basically gear inches times 0.3 gives speed in mph at 100rpm, varies with exact tire size. This was how it was done before such a thing as a 'cycle computer' existed. I am a Luddite and proud of it and still do exactly this. A computer would look all wrong mounted on handlebar of a 1950 Bates.
GPS? I have seen riders crash into parked cars while involved in GPS.