First way: you hunt those Digimon for Gabumon mercenary eggs. You won't get anything from a Digimon if your own hunting partner is 6 levels or more above the ennemy.
Gabumon Snowstorm Village Levels: 30-33
Garurumon Wind Valley Levels: 26-30
Garurumon Snowstorm Village Levels: 33-36
WereGarurumon Oil Refinery-1 Levels: 33-37
WereGarurumon Snowman Village Levels: 39-43
MetalGarurumon Distorted Data Village Levels: 57-61
You can also summon Gabumon with Monster Cards lv1 (mc1), Garurumon (mc2), WereGarurumon (mc3), MetalGarurumon (mc6).
Second way: you buy those eggs from Kamemon's Core Shop (price:5cores), Angewomon's Attribut Crystals Shop (not sure about this one) or from another player (price:12~20Mega on Beelzemon server).
Third way: you buy a Gabumon reinforced mercenary egg in the Cash Shop (70Silks for a random level egg)or from a player (Beelzemon price:700M~1.2Tera for a phase3 egg, 8~14Tera for phase4 and more than 25Tera for phase5).
You need also beast datas to hatch it, 5 per try: if you try normal eggs, you'll need many (the success is not guaranted). Those datas are in magic data beast eggs (those with a scanning price of 125bytes), they are drop from any beast Digimon.