I've bought this guide every year since the early 1990s. At that time it was unfailingly reliable, and included all sorts of pubs from basic drinkers to establishments which were almost hotels.
Over the years, as food has become more important to pubs, I suppose, the Good Pub Guide has become more like a good food guide.
There are many pubs in Britain serving very fine food, from hearty pub grub, through to almost haute cuisine. Unfortunately there are also very many pubs whose chefs are better at writing a complex menu than they are at cooking at a decent level.
The Good Pub Guide seems unable to distinguish good food from menu pretension, and as a result, I have eaten some truly hopeless meals in some of their most highly rated recommendations.
To be fair, you can completely rely on the guide to take you to pleasant and welcoming pubs, but if you intend to eat,and spend more than a few pounds, I would cross-check with a proper food guide.