Its so weird that as I'm playing a new game I was thinking the exact same thing. The first GA is easy to get, but the second takes a CRAZY amount of GAP in comparison.
This is probably an
unnecessary idea that would end up affecting other aspects of balance, but
what if GA were changed to bring the most impact at the beginning of the game rather than at the end? Usually the boon comes from achieving perma GA's throughout the late game, when you're already generating massive yields -- a reason that largely contributes to the final eras feeling the fastest -- but at the first occurrence in the Ancient era, those % bonuses are trivial and practically wasted.
I think it'd be interesting (and better mouthfeel, to avoid situations where a player is actively avoiding GA in the early game) thematically and gameplay wise, to have GA operate with maybe some kind of era scaling or diminishing returns. Historically, I'd say we naturally envision a GA bringing more presence, prestige and '
unph' the earlier it was in the timeline of events, e.g., the Romans, Greeks, Persians, Egyptians, etc, have such impressive and glorified regimes in comparison to others over recorded history, in part due to how much longer ago their respective GA's occurred.
If GA currently grant 20%

+

, we could possibly try something simple like:
- Ancient / Classical = 40%
+ 
- Medieval / Renaissance = 30%
- Industrial / Modern = 20%
- Atomic / Information = 10%
(idk if this influences game speed at all, as I only play standard)
Even with this adjusted new system in place, the % bonuses would still bring ample impact for civs with large base yields in the late game. Late game is also when other modifiers that extend GA length (Ivory monopoly, certain policies, etc.) will have already been established, so although the GA's would not have as high base % modifier as the early eras, their increased lengths of turns while active would still keep them formidable.