Swapping G8 Halogen Under-cabinet fixtures for LED
Are you correct in the questions?  Should it be G8 20 watts not 20V.
To replace the G8 20 Watt/120v build it is a direct straight replacement.  Since you did do the swap, but it is flicking, there could be two issues


The replacement LEDs were not dimmerable and you have a dinner in the switch
The replacements are dimmerable, but the dimmer is not rated for LED bulbs.