Dynamos produce AC which will need rectifying, smoothing and then regulating. The SON28 hub produces at least 40V at high speed with low load (which a GPS is) so you’ll need some resilience.
I have (for Rob C) a working solution whereby I charge the AA’s directly inside the etrex and do away with the external cache batteries. Works on the bench and charges from about 6mph upwards.
A sensible sized capacitor is not enough storage to power the GPS for more than a seconds.
With a USB cache battery the problem I found was that they charge at 500mA (2.5W) at slow speeds (less than about 13mph) the hub can’t produce 2.5W so what happens is the cache battery is charged in pulses as it trys to charge, the dynamo voltage collapses and the charge ends and repeat.