Nerfing non-legendary Pokemon?

Yes

They normally only nerf stats if their original total ends up above 4000cp, then it's a 9% nerf.

As such, Rampardos (3179cp) and Garchomp (3823cp) should be spared a CP nerf, but may end up being constrained by weaker movesets.

Regigigas has a starting CP of 4955 so will definitely face a cp nerf, and likely some kind of weakened moveset to account for the Slow Start ability he has in the main game that limits his potential, similar to what they did with Slaking.

Did you just call Regigigas non-legendary?

Regardless I dont see it happening to non-legendaries (Regi is a legendary, reeeeeeeeee). Slaking was nerfed due to its weird conversion into Pokemon GO. There isn't anything like that in Gen 4.

Alot of people get the idea that Regigigas would be overpowered without Slow Start. This is not the case. Regigigas has stats that are on par with other legendaries. It's ability was implemented in the main series games as a way to make it "balanced" for tournament play. The end result is a Pokemon that is gimmicky to use and outclassed by many other "normal" Pokemon.

Niantic has the opportunity to make Regigigas a viable legendary Pokemon, but not if they cripple it. A Regigigas with a moveset of Rock Smash and Focus Blast (like the Regi trio has) would make for a solid fighting type attacker.

If it had the 9% nerf applied, it's stats would look something like this:

Atk: 286
Def: 210
Stam: 200

Not much different than Kyogre or Groudon, when you compare them side by side.

As for Rampardos, it is a glass cannon in the main series games and will remain so in GO. 295 attack is a lot for a normal Pokemon. If anything, I can see them giving it a weak moveset.

Garchomp is fine though. It's a pseudo legendary, like Dragonite, TTar, Salamence and Metagross.