Be able to execute function while player is in specific radius of part ?

Yep, as the ttle says. What I am trying to do here is to make script which whill do following:

• When players enters radius 5 studs of specific part, he wil be able to use function executed by pressing keyboard button. Any ideas ? If you still don't get the point, keep reading, otherwise there is nothing important let's say that i have barrel, when players walks up in radius 5 studs of that barrel, he will be able to press key to open GUI so he can loot contains of that barrel.

Anyways, thanks.