A five letter word that is the opposite of the word "gradual" and contains a second letter of "l" is a popular word game answer. The five letter word being sought is "fleet."
Sudden
The opposite would be sudden or abrupt, so the word could be quick, rapid, or swift.
A five letter word for the opposite of lower is upper.
The opposite would be sudden or abrupt, so the adjective could be hurried or instant.
A five letter word meaning the opposite of hurry would be dally. There may well be others
The opposite of sudden is gradual.
night
front
plain
Sharp
awake