Gary Hudston wanted to propose to his girlfriend, so (naturally) he contracted with a skilled level-developer to make a custom Portal 2 level that led to a wedding chapel in which GLaDOS (the AI villain of the series) popped the question. And naturally, once Valve (the company that makes Portal) got wind of the project, they authorized the production of custom audio from Ellen McLain, the voice actor who does GLaDOS in the game. And naturally, the developer made the level available so that you could play it too (but that doesn't mean you're getting married to Gary). Ah, romance!
Proposal by Portals! (youtube.com)