Gross pay is everything your employer agrees to pay you for a period before anything is taken off it. It is the number on the contract and in the advert, the first line of a payslip, and the base from which most deductions are worked out. Overtime, bonuses, commission and holiday pay all add to it in the period they are paid.
What arrives in your account is net pay, and the gap between the two is income tax, social contributions and anything you have agreed to have paid out of salary, a pension being the usual one. A UK payslip has to show your earnings both before and after deductions, as GOV.UK explains, so both figures should always be on it.
Gross pay is not always the base for every charge, though. In the United States you pay Social Security at 6.2% and Medicare at 1.45% of your wages, according to the IRS. On $75,000 that is $4,650 and $1,087.50 before any income tax is counted. A 401(k) contribution then lowers the wages that federal income tax is charged on, while those two charges stay where they were, so the same gross figure can be taxed on two different amounts.
Lenders look at income too, and the debt-to-income calculator shows which countries assess it before tax and which after. The gross figure is also what you type into the take-home pay calculator, which works down from it to what you keep, and into the salary to hourly calculator, whose results stay gross from start to finish.