I'm planning to pour gasoline all over the flower garden.
Thats horrible. Why would you kill the flower garden by pouring gasoline all over it?
There are some flowers I don't like.
There are other ways to remove plants from your garden. Get some gloves or natural plant spray to safely uproot the plants instead.
I want to watch it burn slowly, then I will replaced them with pretty flowers again.
If you pour gasoline on these flowers and set them on fire you will ruin the soil. Flowers won't bloom unless they're carefully cared for. You'll end up having to do more work by replacing all the soil and virtually starting from scratch.