'has been gradually established' is correct and usable in written English.
It is generally used to describe something that has become firmly entrenched over time. For example, "Over the past decade, the company's reputation for excellence has been gradually established."
Given such a network of relative ages, a chronology of events has been gradually established in which the relative time of origin of various geologic features is known.
A comprehensive rockburst prevention and control system has been gradually established.
After these works, researchers dedicated themselves to the discussion about the electronic and magnetic properties of graphene with a topological line defect; the line defect-based electronics has been gradually established [33 36].
The theory of electron transport mechanism in mesoscopic systems exhibiting quantized conductance has been gradually established through a time span of decades from the suggestion of the prototype by Rolf Landauer in 1957 [198] to the publication of the book "Electronic Transport in Mesoscopic Systems" by Datta in 1995 [143].
The functions of the Notch pathway in cancer formation have been gradually established, and recent data have also implicated a role for Notch signaling in GSCs [ 12].
In the last few decades, the concept and the validity of maternal near miss have been gradually established, but controversies still exist about the criteria used to define the cases.
For example, data standards have been gradually established for the microarray (33, 34) and proteomics data (34); various software are also available to help integrate the diverse and standalone bioinformatics software tools for a simpler workflow (35 40); and DDBJ, GENBANK and EMBL, the three major molecular databanks have been exchanging and synchronize their data on a daily basis (41 43).
Milad and his friends are a human symbol of the dichotomy which has been gradually become established in Europe since the migration surge began: the deserving refugee fleeing war and persecution versus the opportunistic economic migrant who just wants a western lifestyle.
They are serious and, who knows, they may even be right given the ludicrous way US copyright law has been gradually extended to 95 years after the copyright was established, thanks to the Sonny Bono copyright extension act of 1998.
