If you are going to use gravel from an established & healthy tank then you don't need to any ammonia.
not true. while there will be some bacteria in the gravel, a handful is not nearly enough to add your fish to and expect everything to be peachy. most of the bacteria is in the filter media and other high-oxygen places. there is some in the gravel, but less, and only on the very top layers exposed to circulation. unless you take all the gravel and all the filters from one tank and move it to another, you'll still have some kind of cycle.
the point of seeding a fishless cycle is to get a head start instead of waiting for some good bacteria to wander in on their own. you still need to feed with ammonia over a period of time in order to raise the amount of bacteria up to a high level where you can add all of your fish at once and the waste needs will be taken care of. just a handful of gravel isn't going to do the job.
seeding = adding bacteria from another source.
adding ammonia = adding food for the bacteria.
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