The poor chap probably went from thinking he got a free upgrade to being offloaded.

I'd also have no idea what GGL meant if I hadn't frequently visited this forum.

And even as a frequent flyer, I think the first time I travelled BA in Europe I thought to myself upon boarding, oh this is a strange plane it only has economy. I didn't immediately realise that a different cabin could have the same seats, because I was used to flying domestically in Australia and the US where the first few rows are larger seats in 2-2 or 2-1 config.