Hello Tobin,
You're right not to wish to be brainwashed (by anyone) ... Even the Buddha taught, "do not believe anything, no matter where you heard or read it - even if you heard it from me, unless it agrees with your own common sense and reason". I think this is a very important teaching - it's why I wrote, "... check things out for yourself with open eyes and an open mind" - my opinion is just as likely to be mistaken as anyone else's

The three year retreat centre at Gampo Abbey is called
Söpa Chöling, has its own website. The Program is
not completed in one three year block but in stages. You need to have a number of Shambhala programme under your belt before you would be granted entry. It is a Vajrayana training in the Tibetan tradition so you would certainly need to familiarise yourself with what this means before even contemplating going down that route.
Tibetan Vajrayana (actually Tibetan Buddhism in general) is a mixture of Buddhist, tantric Hindu and Bön teachings. This includes deity worship and practices and quite a few other things that personally I place in the category of "religious cultural baggage" or agglomerations/accumulations to the teachings of the Buddha. It is also a path where you give your self entirely into the hands of the teacher/master.
Two of the most famous Tibetan masters to come to the west have found themselves amidst great controversy, Sogyal Rinpoche and Chogyam Trungpa Rinpoche (founder of Shambhala). Both had great influence and helped spread the Dharma ... both gained reputations for sleeping with their students and Trungpa died early of sclerosis of the liver due to alcoholism. He handed the Sangha to his Dharma Heir who, infected with HIV, thought his "high spiritual attainments" meant he could not transmit the disease to others, so he followed in his master's footsteps, slept with a bunch of students and consequently transmitted the disease to them resulting in illness and early death.
These truths get to the bottom of an issue with the master-centric forms of Buddhism in general and also Vajrayana: the ego often seems to blossom and enlarge, not wither and die, despite seemingly gaining high levels of wisdom on the way. This was something I saw for myself in the eighteen months I spent in a Shambhala centre: lots of "old", I.e. long-term, students with lots of "issues". The most common was alcoholism and, it seemed, encourage by Trungpa's example and by his teachings on "mindful drinking".
But, as I said before, everything changes - and it was certainly not all bad. Sakyong Mipham Rinpoche (Trungpa's son and Shambhala's current leader) took action against one of the senior Acharyas, instructing him to stop sleeping with students, showing a level of wisdom and trustworthiness.
I met very many wonderful, generous, spirited and kind people from the Shambhala Sangha in my time there, and though I officially "disengaged" from the Sangha in 2008/9 I'm still in contact with some good people there.
The base concepts are simple and wise: enlightened society and "basic goodness", a term I suspect Trungpa coined in direct opposition to the Christian concept of original sin. But IMHO it's a bit of a "rabbit hole" and especially if you get into the tantric/Vajrayana side.
I hope I have answered your questions in a way that is educational.
Kindly,
Matthew