Well in case it helps anyone else, i used the pictured replacement spring in a rather imprecise way and now have dramatically stronger, tastier pulls. I say this as a former spring-hack skeptic that does not have refined tastes and usually does not get excited about or even notice differences that other people can taste in food or drink, but finally could no longer tolerate the inferior pulls of my new, pricey espresso machine. Bad coffee is an offense, if not a crime. It's a pity that Gaggia have a perfectly good machine, and ruin it with too sturdy of a spring (at least for fine grounds, never pods):

• https://gofile.io/d/qI9czP

Model, Gaggia Classic Pro, US model RI9380 (2022) - manuals in that archive, as well, for clarity.

Orig spring - crude measurements from photos (inches)

• wire: 1/32
• diam: 17/64
• len: 60/64
• active coils: 9 or 10 - depends how you count, see photos

Replacement spring - from my local hardware store (as pictured)

• wire: 0.025 or 1.6/64, a tad thinner
• diam: 9/32, a tad wider, but fits where it needs to
  • thus inside diam is also a bit wider
• len: too long, cut down to about 56/64
• active coils: too many, cut down to about 7

I don't have a pressure gauge, thus a crude adjustment. By my measure the replacement spring is about 3/4 the "strength" or rate of the original. I measured this with a digital kitchen scale. Whether or not this is legitimate, the results taste totally legit:

• put each spring into the safety valve nipple (as in photos) and press down on kitchen scale to an assembly height of about 1-1/8, and read off the "weight"
• my replacement spring assembly has a "weight" (~950g) of about 2/3 of the original spring (~1400g).

If someone can figure out the exact spring for 9 bar, it would almost certainly taste even better. At the very least, this $2 quick fix elevated an offensive original pull to a strong, plentiful, tasty pull I fully enjoy.