"essential gain" is a perfectly acceptable phrase that can be used in written English.
You can use it to refer to something important that you have gained, as in the following sentence: My hard work and dedication to my studies paid off, giving me an essential gain in knowledge.
Did it even register in his heart that women (and men) came out in droves to assert their presence, to give voice to their lives and their legitimate fears that the new administration may be willing to strip us, as a class, of so many important, essential gains?
Clinics for adolescents are keenly conscious that the promise of confidential care is essential to gain and hold their young patients' trust.
She writes: Clinics for adolescents are keenly conscious that the promise of confidential care is essential to gain and hold their young patients' trust.
Research involving genetic modification of human embryos, though controversial, is essential to gain basic understanding of the biology of early embryos and should be permitted, an international group of experts said on Wednesday.
In light of those trends, Verizon deemed it essential to gain control of its biggest business, which in the most recent quarter accounted for $20 billion of the company's nearly $30 billion in revenue.
Validation of these computational models is essential to gain confidence in the codes.
Validation of these numerical codes is essential to gain confidence in the codes.
But it's also essential to gain familiarity with technology and working with data — and it's something they'd never do in school.
Characterization of their structural/dynamic features is essential to gain in-depth insight into structure-activity relationships.
It is essential to gain insight into the available solution space to generate trajectories that meet a variety of constraints.
