The phrase "a source of gain" is correct and usable in written English.
It can be used when discussing something that provides benefits, profits, or advantages in various contexts, such as business, investments, or personal development.
Example: "Investing in renewable energy can be a source of gain for both the environment and your financial portfolio."
Alternatives: "a means of profit" or "a source of benefit."
It has also been reported that while young men engage in substance use as a way of doing gender, substance use among their female counterparts is a source of gaining cultural capital [ 47].
The report said there was "widespread evidence of collusion" between the government and "A.Q.I.M. and organized crime," and suggested that a portion of hostage ransoms for kidnapping victims was "possibly, a source of financial gain for senior officials".
In contrast, rising land prices have provided a source of financial gain to Chilean plantation owners that few may have anticipated.
The effect of today's energy boom on broader productivity remains to be seen, but it could prove a source of further gains.
Even men such as the English geographer Richard Hakluyt were not certain that the Spanish colonization experience could or should be imitated but hoped nevertheless that the English colonies in the New World would prove to be a source of immediate commercial gain.
Until unions become both advocates and bargaining agents, something which they have been unable or unwilling to do so far, they cannot count on the emerging work force as a source of significant membership gains.
We argue that this zero-beta property of scientific FDA risk could be a main source of gains from trade between issuers of FDA hedges looking for diversified investments and developers looking to offload the FDA approval risk.
On top of that, as US data gurus realised in the last presidential campaign, the social network is an unrivalled source of gaining information on voters.
Interestingly, in his comparative study on the foreign policies of Kazakhstan and Uzbekistan on the one hand and Kyrgyzstan on the other, McGlinchey (2012) came to the conclusion that sometimes autocratic leaders may be more supportive of the West than democratic ones, which happens exactly because of the formers' need for an additional source of gaining domestic legitimacy.
