Before you plant any thing else in that space, confirm how big the holly bush will be at its mature size. I think if you plant annuals around the holly bush until holly covers the gas tank you will be happy with the final results.

Geographic location?

Circling the gas feature with some plants and then having bare space around those plants is calling attention to it rather than obscuring it.

Herbaceous perennials with tops that largely disappear during winter will provide no screening between fall and spring.

You could try ornamental grass. Many remain tall and dense throughout winter, but when you prune them in the spring they won’t be providing coverage until they fill in again. Also, you probably already are aware, but holly can have some rather prickly leaves. Make sure that if this a tank that needs to be accessed then you don’t want the holly too close or interfering.