"has been gradually expanded" is correct and can be used in written English.
It is used to express a gradual increase in something over a period of time. For example, "Over the years, the company's customer base has been gradually expanded."
The discard ban originally covered cod and haddock in the economic zone north of 62°N, but the ban has been gradually expanded so that today it is prohibited to discard most species of fish in Norwegian waters.
This cash supplement to wages, delivered in the form of a tax refund, was created during the Ford administration and has been gradually expanded under Republican and Democratic presidents.
It started out on mobile devices and tablets but has been gradually expanded to Roku, Apple TV, and the Xbox.
Since its inception the workshop system has been gradually expanded to encompass not only advanced graduate students, but less advanced graduate students' MA papers and dissertation proposals, as well as visiting scholars' and faculty works in progress.
Paved during the 1930s, this road has been gradually expanded to four lanes over the course of the 20th and early 21st centuries.
Furthermore, the government announced the extension of a mandatory free education from 12 years to 15 years in 2009.Access to basic education has been gradually expanded to an increasing number of children.
The company, based in Brentwood, Tenn., has been gradually expanding in Europe since late 1996.
In view of this state of affairs, the nationalist parties opened up to another brand of nationalism: economic nationalism, whose support base has been gradually expanding.
But it has been gradually expanding its agricultural business beyond Europe and Russia into other grain markets including Australia, where it started operations nine years ago and where Viterra also has substantial assets.
Stephen Starr, the Philadelphia-based restaurateur who has been gradually expanding his New York portfolio, will add a version of El Vez, his Mexican restaurant, to New York's financial district late next year.
Allan Houston, who began a quiet apprenticeship in the Knicks' front office two years ago, has been gradually expanding his role and preparing for an eventual future as a general manager.
