1st and 3rd Generations are at the same time.This is confusing D: Who's to say that DPP is at the same time as GSC? The red gyarados could have been sighted much before the protaganist in GSC battles/defeats it at Lake Rage.
2nd and 4th Generation are also concurrent.
3rd Generation includes FR/LG, so R/B/Y, R/S/E, and FR/LG all occur at the same time.
HG/SS is part of the 4th generation, so G/S/C, D/P/P, and HG/SS are in the same time period (Which happens to be three years later than the 1st and 3rd Generations).
This has been confirmed by everyone with an IQ over 48, and then beaten into the ground with logic.