I visualise the gum weights like this:
When you hit the boon bar and push boon points to a boon, the boon is selected from a deck of cards (boons). By default the deck has 1 card for each boon it’s possible to get at that moment*. A single active gum adds 10 extra cards for the boons it targets, so a single effect gums adds 10 cards for damage, and 10 cards for crit effect. Three active effect gums would add 30 cards to both boons.
Then a single card is drawn and the points go to that boon.
*If the 3 slots are already full, the only possible boons are those 3 boons, but gums still effect the weights of them.