Are there any local agencies in Darjeeling that do the Goechala Trek?

Yeah, I know one. If you wish to Go on the Goechala trek, you can obviously choose Glacier Treks and adventures. They are a local trekking company and Government registered company. Let me help you give the reasons why you should choose them for going on Goechala Trek.
They are the Government registered company
They believe in eco-friendly practiced
Quite affordable as compared to others
The trek guide is professionals to guide you through the trek
They constantly judge the weather conditions for your safety
Comfortable camp locations
Yak, porters, and kitchen staff assists you on the trek
According to me, they are probably the best trekking company in Sikkim, but you should do your research before finalizing any guide.