At 20min, Gathering Storm is roughly equal to any other rune you could have taken (522g - stopwatch is worth 600g and most other runes are worth either around 300-400g or start out around 100g and scale to around 500-550g).
At 50min it's worth over 2600g in stats.
Now, I get that this rune takes some time to come online. It doesn't offer anything outside of raw stats (i.e. nothing you can't make up for in items if your other runes get you ahead, unlike the runes with unique effects that you can't buy). It still seems way too strong though. It's also like, at 39min I get wrecked in a 1v1 against someone, then at 40min I can wreck them 1v1. It doesn't seem very transparent.
I think the rune could be balanced in a state where at 10/20min it gives almost nothing worth in stats, it starts to turn on at 30min, and then at 40/50min onwards it can really be the strongest one. Although even if the numbers are balanced, I think we need a clear visual signal that someone has this massive storm gathered on them. As far as I know, there's no way for my opponents to tell I even have this rune, so I'm sure it feels pretty bad to suddenly see my AD jump by 32 in the middle of a fight because we just happened to cross the 40min mark.
As an aside: Good job on the summoner spell swap. The cooldown is set to the maximum of the cooldown of the spell you got rid of, and 30 sec (from what it looks like, and maybe different for smite). That's pretty sweet, as it would have been busted to let me swap an available summoner for one at no cooldown. Even with the 30sec cooldown I'm catching people super off-guard by swapping my smite for TP and teleporting behind them for ganks. If anyone could just immediately grab TP at the fountain that would be busted - so good foresight on that!