You have the ability to randomly get a single 'item', 'power' or 'person' of significance in fiction per 'roll'. They generally manifest in form of cards which you can use to summon them, however you may choose how they manifest. Other than the above specified items, you can also get power boosting and weakness removal cards which you can use for both yourself and your summon. The rarity of the card determines how bad a weakness can be removed. Any changes to the world caused by this power is retroactive and would not cause any suspicion among the residents of the world. Any powers or items you get work no matter what universe you are in. Similarly, people summoned by the gacha also have their power unchanged. The people summoned by the Gacha while not completely loyal to you, are willing to work with you if you don't go too much against their principles (your relationship with them is similar to Master-Servant relationship from Fate). You also possess three command seals (refreshed daily) that you can use to give them an absolute order. Any item you get from gacha repairs itself with time if broken, with stronger items taking more time for repair. Any summoned person who died can only be resurrected with the use of resurrection card (that you may get from gacha) or some other resurrection power. This power by itself doesn't have any innate ability to resurrect them. You get a single gacha point daily and can get more when you defeat your enemies, interact with people important to plot and complete or change plot points of your world. The number of points you get varies from one to ten, depending upon the power of your enemies, importance of the person in plot and how major plot points you have completed or change. An example being preventing Scion from destroying earth will give you ten points. Similarly meeting the protagonist will also give you ten points because of their central role in the story. The tiers of your items, summons and powers are: F, E, D, C, B, A, S, SS and SSS with F being Junk items, E being an item like "always filled water bottle" and SSS being top tier divine abilities and authorities (taken into account that you are a non-powered mortal). If you roll five junks (F tier) one after another, you get a pity point, preventing you from getting a junk in the next roll. Your power generally manifest in the form of a HUD, where you can spin a wheel to get your items. Or you can freely design how it appears.
(You may choose to not absorb the power you get from Gacha and give it to someone else. You may also choose to give the items you get to others. However, any power or item you get from this Gacha, will never be able to hurt you. The rate of getting items from higher tiers decreases exponentially from F tier, with SSS tier having the lowest probability. The items and power you get from the Gacha varies depending on your base power level, such that no item or power is totally useless to you (except if you roll F tier). The probability of getting same items or powers is extremely low, and in case you get the same item or power, you can sacrifice one of them to strengthen the other.)