Just understand that if you have specific goals, specificity is going to be number one as I say again and again. Want a big chest, specifically work that muscle group frequently and overload. Want to be a competitive GAA player, train that specific sport frequently. This piece is for someone who just enjoys training and liked to have an array of skills in their arsenal, you don't always have to have a specific goal, sometimes it's enough to train for the love of it and try new things.
GPP ia important for everyone as strength is at the base of everything, before becoming proficient in any sort of activity, having general strength will serve you. If you are beginning your journey in the gym, and you begin training specifically for your sport, without learning the proper mechanics, becoming a proficient mover or building up your general strength, you are going to regress down the line. One must become a strong generalist before becoming specific. general physical preparedness allows you to build that base, figure out what you enjoy, what you respond best to and then down the line, if you wish to specialise, it will be a lot easier for you. If you are young or train younger people, build their general skills through different modalities and as a result they will be more confident when it comes down to specialising.
GPP can be defined as a lot of things, but I like to think of it as being an all rounder. For example, Bbeing able to squat bench and deadlift heavy relative to your body weight, being able to run a 10k, being able to play a game of astro turf with your friends and even be able to jump in on a CrossFit class with some skill work. Being able to perform all of these different tasks is just as impressive as a specialist at the top of their game.
For those of you who will never specialise or simply don't have the interest in specialising, I advise you to train using the GPP method. This will allow you to keep training exciting, constantly learn new movements/skills, get different adaptations and improve your tool box.