GPP - Introduce Novel Functional Stimulus & Get Something Done
This is what at least some of your GPP should resemble.
There are a lot of quality gym-based exercises that are effective for shoring up weaknesses and imbalances and preparing your body to do what you ultimately need it to do as an athlete (and person).
But nothing will stimulate the same way that manual labor does. There is an inherent irregularity even in repetitive motions (e.g. shoveling); there are uncontrollable and unpredictable elements like uneven ground and restricted spaces that force unusual body positions; there are plenty of things to grab and lift and carry that are unbalanced, awkward and don’t have convenient handles; and there are challenges to not just trunk and total body strength, stamina and stability, but to coordination and skill that provide totally novel motor learning experience.
Additionally, you get the satisfaction and the lessons attendant to completing worthwhile physical tasks. And hopefully, you get to be outside and reap the benefits of fresh air, sunlight and the long list of positive physical and mental affects of outdoor exposure.
If you don’t have opportunities at home or with friends or family for this kind of work, go volunteer with an organization like Habitat for Humanity - they can always use help with unskilled (read: hard) labor. If you can’t make that work or just really hate being a good person, get creative and figure it out - find a pile of rubble on the side of a back road and lift, carry, throw, stack, and move back if you have to.
And remember that as you get older, you have to actually be useful, not just good looking.