If I find a shiny rock and sell it to you for $100 and you find a fossil and sell it to me for $100, that's $200 added to the GDP. If we trade rather than pay each other, then it's $0 added to GDP. The same goes for services. If I mow your lawn, paint your house, hypnotize you to quit smoking, or sing you a song - all of those things (are included in GDP.

When you pay for entertainment it's 100% included in GDP the same way as when you pay for somebody to inspect your car.