B1. to get something that is useful, that gives you an advantage, or that is in some way positive, especially over a period of time: The mayor has gained a lot of support from the teacher's union.

What do you mean by gaining?

verb. gained; gaining; gains. Definition of gain (Entry 2 of 2) transitive verb. 1a : to acquire or get possession of usually by industry, merit, or craft gain an advantage he stood to gain a fortune.

Has gained or have gained?

I have gained means you are referring to the experience you gained until today. I gained is referring to the experience you gained in the past. Hardly somebody would think you didn't gain any experience, recently, if you say I gained.