Gravity Falls and the Twin Paradoxon
The fact that Grunkle Stan’s alleged twin looks around 20 years younger than him, reminded me of a thought experiment related to Einstein‘s theory of relativity: In short, Einstein’s theory proposes, among other things, that if someone travels at extremely high speeds, the time for him passes at a far slower speed than for people that stay behind. So if you’d take a set of twins, and sent one of them out into space at a velocity near the speed of light, while the other one stays on earth, the one on earth would age at a normal rate, while the other one in space would age far slower, and arrive on earth much younger than his brother.
See what I’m getting at? We do not know where the portal leads, but I think that instead of Time stopping at that place, it might be that travelling to that place could happen at such a high speed that for Stan’s brother just a few years passed (if any), while for Grunkle Stan, all the events were 30 years ago.
This might also explain why Stanbro looks so pissed, since for him the events happened pretty recently and he didnt have enough time to process/forgive/etc.