The difference between Gain and Increase

When used as nouns, gain means the act of gaining, whereas increase means an amount by which a quantity is increased.

When used as verbs, gain means to acquire possession of, whereas increase means (of a quantity, etc.) to become larger or greater.

Gain is also preposition with the meaning: against.

Gain is also adverb with the meaning: straightly.

Gain is also adjective with the meaning: straight, direct.

check bellow for the other definitions of Gain and Increase

Gain as an adjective (obsolete):

Straight, direct; near; short.

Examples:

"the gainest way "
Gain as an adjective (obsolete):

Suitable; convenient; ready.
Gain as an adjective (dialectal):

Easy; tolerable; handy, dexterous.
Gain as an adjective (dialectal):

Honest; respectable; moderate; cheap.

Gain as an adverb (obsolete):

Straightly; quickly; by the nearest way or means.
Gain as an adverb (dialectal):

Suitably; conveniently; dexterously; moderately.
Gain as an adverb (dialectal):

Tolerably; fairly.

Examples:

"gain quiet (= fairly/pretty quiet)"

Gain as a noun:

The act of gaining; acquisition.
Gain as a noun:

What is gained.
Gain as a noun (electronics):

The factor by which a signal is multiplied.

Gain as a verb (transitive):

To acquire possession of.

Examples:

"Looks like you've gained a new friend."
Gain as a verb (intransitive):

To have or receive advantage or profit; to acquire gain; to grow rich; to advance in interest, health, or happiness; to make progress.

Examples:

"The sick man gains daily."
Gain as a verb (transitive, dated):

To come off winner or victor in; to be successful in; to obtain by competition.

Examples:

"to gain a battle; to gain a case at law"
Gain as a verb (transitive):

To increase.
Gain as a verb (intransitive):

To be more likely to catch or overtake an individual.

Examples:

"I'm gaining (on you)."

"[[gain ground]]"
Gain as a verb (transitive):

To reach.

Examples:

"to gain the top of a mountain"
Gain as a verb:

To draw into any interest or party; to win to one's side; to conciliate.
Gain as a verb (intransitive):

To put on weight.

Examples:

"I've been gaining."
Gain as a verb (of a clock or watch):

To run fast.

Gain as a noun (architecture):

A square or bevelled notch cut out of a girder, binding joist, or other timber which supports a floor beam, so as to receive the end of the floor beam.

Increase as a verb (intransitive):

(of a quantity, etc.) To become larger or greater.

Examples:

"His rage only increased when I told him of the lost money."
Increase as a verb (transitive):

To make (a quantity, etc.) larger.
Increase as a verb:

To multiply by the production of young; to be fertile, fruitful, or prolific.
Increase as a verb (astronomy, intransitive):

To become more nearly full; to show more of the surface; to wax.

Examples:

"The Moon increases."

Increase as a noun:

An amount by which a quantity is increased.
Increase as a noun:

For a quantity, the act or process of becoming larger
Increase as a noun (knitting):

The creation of one or more new stitches; see .