"gradually waned" is correct and usable in written English.
Example sentence: As the sun set, the intensity of the light gradually waned.
Over time, though, the practice gradually waned; by the nineteen-nineties, it was almost completely gone.
After the Mongol invasion of Baghdad in 1258 and the Spanish reconquest of Granada in 1492, the magnificence of Islamic culture gradually waned.
1220), the power and vitality of the kingdom gradually waned until finally, after the armies of the Tai state of Ayutthaya (Ayudhia) captured and sacked Angkor in 1431, the city was partially abandoned.
Chinoiserie gradually waned during the 19th century, when the appeal of China and East Asia had to compete with other exotic tastes, such as the "Turkish," the Egyptian, the Gothic, and the Greek.
Activision has developed a great reputation for launching billion dollar franchises, but Call of Duty's popularity has gradually waned over the last three years, while Skylanders is a series that is suffering a significant fall in popularity.
With no sustained training and little technical support, the initial eagerness of most faculty members gradually waned.
Abortion opponents' numbers may be gradually waning, but they are winning all the same.
As democracy takes root in Taiwan, the practice of handing over cash is gradually waning.
With his once phenomenal popularity gradually waning, Mr. Putin will have to find other ways to guarantee his legitimacy.
It's evening; there are no windows in the meditation room, but there's ambient light, gradually waning.
If anything, the language becomes even more exuberant but its impact gradually wanes, and in her effort to sustain it, Russell loses a handle on the plot.
