You must fight those ennemies located in those areas: Gabumon Snowstorm Village Garurumon Wind Valley Garurumon Snowstorm Village WereGarurumon Oil Refinery-1 WereGarurumon Snowman Village MetalGarurumon Distorted Data Village Gabumon Summoned by Monster Card Lv1 Garurumon Summoned by Monster Card Lv2 WereGarurumon Summoned by Monster Card Lv4
After a while, some will drop hatchable eggs. Those summoned by Card are more subjects to mercenary egg dropping. Sometimes you'll get a black Gabumon egg, but this is pretty rare. Note: the Digi you use to fight must be within 10levels around the opponent's level.