big, considerable, dramatic, enormous, huge, impressive, major, real, significant, spectacular, substantial, tremendous
This change in the tax system will mean big gains for some companies.

solid, strong
The party has made solid gains in all areas of the country.

tangible
minimal, modest, small
double-digit (business, esp. AmE)
rapid, steady
immediate
future
potential
long-term, short-term
annual, daily, monthly, etc.
windfall
net, overall
The party made an overall gain of 39 seats.

ill-gotten gains
She tucked her ill-gotten gains into her purse and left.

hard-won
personal, private
They were found to have used the investments for their private gain.

commercial, economic, financial, material, monetary
There will be no financial gain for mothers from this new system.

electoral (esp. BrE), political
the right made huge electoral gains

military, territorial
fat (AmE), weight
She was upset by her recent weight gain.

efficiency, productivity
There is still scope for efficiency gains.

fitness

achieve, make, reap
He is already reaping political gains.

maximize, minimize
increase, reduce
consolidate
There needs to be joint political action to consolidate the gains of the elections.

bring (sb), produce, yield
Better workplace design can bring real gains in productivity.

seek
Volunteers give their time without seeking any monetary gain.

show
August showed a gain of 144 000 jobs.

offset
The party has an uphill battle to offset the gains made by other parties.

outweigh
The costs far outweigh any efficiency gains.

reverse, wipe out
War and poverty have reversed previous gains in children's health.

for gain
It's amazing what some people will do for gain.

gain from
€3.9 million gains from the sale of stock

gain in
Last year there was only a modest gain in earnings.

quickly, rapidly
gradually, slowly, steadily
His ideas gradually gained acceptance.

immediately, suddenly
eventually, finally, ultimately
successfully
enormously, immensely, significantly
The industry will gain enormously from the new proposals.

financially

stand to
The company stands to gain a lot from this government plan.

expect to, hope to
What do you hope to gain by this action?

attempt to, seek to, strive to, try to
Protesters tried to gain access to the palace.

fail to

from
We all gained a lot from the experience.

have everything, little, a lot, nothing, etc. to gain, have everything, little, a lot, nothing, etc. to be gained
Why not give it a go? You have nothing to lose and everything to gain.
I don't think there's anything to be gained from this course of action.

Gain is used with these nouns as the subject:
index, watch

Gain is used with these nouns as the object:
acceptance, access, acclaim, admiration, admirer, admission, admittance, advantage, affection, ally, altitude, appreciation, approval, assurance, attention, autonomy, backing, bearing, benefit, clarity, clout, command, compensation, competence, composure, confidence, consent, control, convert, cooperation, credibility, currency, custody, degree, diploma, doctorate, dominance, edge, efficiency, employment, entrance, entry, exemption, experience, expertise, exposure, fame, familiarity, fat, favour, first, following, foothold, footing, freedom, fulfilment, height, hour, impetus, impression, independence, influence, information, initiative, insight, inspiration, knowledge, kudos, lead, leverage, licence, livelihood, majority, mark, mass, mastery, maturity, membership, momentum, monopoly, muscle, nickname, notoriety, passage, permission, perspective, picture, pleasure, point, popularity, position, possession, power, prestige, prominence, promotion, publicity, qualification, recognition, recruit, reprieve, reputation, respect, respectability, reward, right, satisfaction, scholarship, seat, self-confidence, semblance, sense, significance, skill, speed, stature, status, strength, summit, support, supremacy, sympathy, throne, time, trust, understanding, victory, visibility, vote, wealth, weight, win