As a a complete novice in the world of espresso, starting with what is apparently a common machine - the Gaggia Classic Pro - I thought it would maybe useful for similar people in my situation to list below some of the things I've found working for me and some of the "a-ha!" moments I've had in this very exhilarating and sometimes frustrating journey.

I am using a Gaggia Classic Pro, without any major modifiers. The only upgrades I've done were:

- Add a proper 58mm tamper
- Buy a very basic WDT tool
- A distributor tool
- A metal funnel for the 58mm portafilter


Dose

Here I've probably lost the most time+coffee, insisting stubbornly to squeeze as much coffee as possible in the standard basket that came with the Gaggia. I tried various coffees, with 21 (yes, 21g), 20, 19, 18g (which should be the basket capacity).
Long story short, the sweet spot for the standard basket on the Gaggia Classic Pro (at least for me) is 16g. With Everything else remaining equal, of course.
With 16g I seem to be able to dial in pretty much any coffee and I get a pretty good shot.

Coffee

I feel pretty stupid recognizing that I knew very little about coffee. Not only espresso, but coffee in general. I wasted a lot of time trying to troubleshoot shots that were just highlighting aromas I was not used to. I got some good advice on this forum and I was able to get decent shots of lighter roasts using 14g dose and a very fine grind. But honestly if anyone is looking for a "classic" espresso shot, a medium-dark roast is all one needs. Right now I get amazing, sweet and powerful shots using this roaster in Montreal - https://www.trafficcoffee.com/products/dante-street

Also what I have noticed is that a lot of the coffees branded as medium-dark roasts are still too fruity for me, so blends marketed as "espresso" were closer to what I would expect in a traditional Italian café. For all the other blends, I've found that I much, much prefer French Press or a Moka Pot in order to enjoy the full spectrum of aromas.

Most, if not all of the challenges around "sour espresso" came from this.

Grinder

I do not have the space for another big machine, I ended up getting the J-Max grinder from 1zpresso. This stuff is seriously good. It's fast, very little static. Before doing that though I believe I wasted about 2 months trying to make it work with my old Krups grinder, which was either too fine or too coarse.

Process

For my fellow newbies outthere, this is what works for me, very predictable now, in order to get a great shot that works as espresso or as base for milk drinks.

16g dose grounded at 1.3.5 to 1.4.5 on 1zpresso
Funnel on portafiler
Vigurously stir with WDT tool
Slowly break up methodically using a circular motion with WDT
One tap to settle
2-3 twists with the distribution tool - I could skip this but I've found it does help a bit making sure the puck is even. To be honest though, the price I paid for the distribution tool is not worth it.
Use the tamper to compress using medium force, nothing too crazy

This gives me about 32-34 g espresso in around 30 sec. For the roast I am using temperature surfing makes no discernible difference so I am not bothering with that.

Single Espresso

I am successfully pulling single espresso shots using the single basket that came with the Gaggia. The dose that seems to work is 9g, everything else I do pretty much the same. I do not have to change the grind size.

Conclusion

Most of my challenges came from not understanding properly aroma profiles and just how diverse the coffee we buy these days can be. The second main challenge was around dose since I was told that the basket size is 18g. Everything else being equal, Gaggia Classic Pro is a great machine making fantastic shots.

Dumitru,

One other thing I'd recommend you "investigate" is temperature surfing... Depending on where you are in the heating cycle when you start your shot, the temperature profile will be different. There are lots of different resources explaining what temperature surfing is but in short what you're trying to do is always start your shot at the same spot in the temperature cycle for more consistent results...

When the heating element has just switched off, that is the point of maximum temperature. Just before the heating element switches on, that's the point of minimum temperature. Time how long it is off and how long it is on and try to consistently start your shot at different points to see what tastes best... If you start the shot too hot, it may be more "bitter" so wait a bit longer after the element turns off to start at a lower temperature... Conversely if you start the shot too cold, it may be a bit more "sour" so adjust accordingly.

You get the idea...

Have fun on the journey!
Josh

thanks for the advice Josh!