m0useanswered grow question 2 years ago I like and have experience with gaia green and enjoy their line up.
a lot of dry amendments are dusty and you will want to wear a mask when mixing into the top layer of soil.
Both brands are very similar and I don't think you'd notice a big difference as long as the NPK ratios are in check.
using both at the same time could be fine, just a waste of money I think if they are mostly the same ingredients.
One note with gaia green I find it lacks in Magnesium and that will need to be added once a month. If you wanted some extra K as well it can be added in with their soluble seaweed extract.
Good Luck!