The Cristiano Garella circle of wineries today includes...
Colombera & Garella, where there is an advantage to be found in the old vines that the Colombera family was farming before the partnership.
Le Pianelle. Cristiano lives at the Le Pianelle winery facility. He is a partner in the winery.
Boniperti in Fara, where Cristiano consults.
Ioppa in Ghemme, where Cristiano has recently started consulting.
Ca' Nova in Ghemme, where Cristiano does some consulting.
Zambolin. Fabio Zambolin is also a partner in Le Pianelle.
La Prevostura in Lessona. Which strike me as good wines.
Pastoris, which is an Erbaluce producer.
And there may be someone that I am forgetting (apologies, if so).
Robert Camuto gives 2013 as the year Cristiano left Sella:
https://www.winespectator.com/blogs/show/id/51358
VLM mentioned several worthy producers already. I would mention that I regard Sperino more highly than he does. I would also just point out that Ar. Pe. Pe. is in the Valtellina, in Lombardia. Also, some people (like Cristiano) do not consider Carema to be a part of the Alto Piemonte. If you are looking for some tasty wine that is not from the Alto Piemonte, Cantine Garrone is a producer to note.
In terms of others to mention back in the Alto Piemonte, Barbaglia is noteworthy in Boca. La Palazzina is noteworthy in Bramaterra. Bianchi in Sizzano. Tiziano Mazzoni makes wines that will appeal to certain folks.
There is a lot happening in the Alto Piemonte, and there are producers that are popping up that I do not know yet. There are also big changes going on at certain wineries, like the recent buy in of Roberto Conterno (of Giacomo Conterno) to Nervi, a Gattinara producer. The Conterno news was the big news locally when I was in the Alto Piemonte recently.
I have been told that Vallana has exited the production of Gattinara.