So gem is bound to character. After you gem the item with that soulstone gem then you can transfer the item to the other toon which you have the soulstone gem also. If you're doing PVP this will be a great advantage for the toon coz giving u +21 damage for a 3 slot mythic/arcane item losing you only 90 hp for 3 normal gem as trade-off,180 hp for maximum trade-off if you have 3 slot grandgem. Thus for one item only, so socketing it into another item will give you more damage. These are my perceptions as of now, I don't want to conclude what are the outcomes. For PVE it's not really a matter for me as a warrior coz I'll let the mage/rouge for damage.