They’re both reusable, but otherwise different.
Oyumaru is like clay when soft and you press it onto the original item (the master), then it is stiff when cool. The master must be sturdy and rigid enough to stand up to some pressure. Oyumaru is better for flat or one-sided items, like a press mold, and is more durable.
This G&G (glycerine and gelatin) material is liquid when hot; you pour it like silicone mold material, and it’s flexible when solid. The master can be pretty delicate.
G&G is great for complex 3-D items, and reproduces fine details. I used a straight pin to hold a master in place; when I cast resin in that mold, the pin is reproduced and I have to cut it off.
Did I mention not to leave it outside on a hot day? I did; the mold softened and slumped while the resin was setting, and I wound up with a weird wavelike item instead of a doll head.