OP, I'm not sure what opportunities there are in MB, but there are lots of landscaping businesses here on Vancouver Island (where the growing season is long and gardens are maintained year round). It's seasonal work, but if you're mustachian and were able to get a good client base going, I would think you'd be able to do pretty well.
I think there is a growing niche in helping people to set up vegetable gardens too (at least I'm hoping long term I might have a kind of homestead-set-up-consulting business), as part of a landscaping business. There's also working in nurseries (again, here on VI, we have a number of excellent nurseries that operate year round and do workshops throughout the year, though of course winter is slower).
There are also lots of urban farmers; I'm sure there are some everywhere; in fact, I think SPIN farming was developed by a couple in Winnipeg or Saskatoon? Out here there are also NGO food security groups starting and maintaining community gardens, and working on a variety of other projects to get fresh food to people who can't afford it. There might be groups in your area too, or you could start one?
The other career I think looks really amazing is horticultural therapy. I don't know if you've heard of this or have opportunities where you are, but this is a diploma program that then enables you to work with people with a variety of special needs using gardening to help them learn life skills and integrate into mainstream society. In my area, google Providence Farm for an example.
I think the MG gardener program also asks you to put in a certain number of volunteer hours; that might give you a better idea of what opportunities are available in your area? On the mainland and the island we have a variety of botanical gardens, experimental farms, large seed companies, small local seed companies, etc that might all be also worth exploring.
Although you might end up needing more than a MG certificate for a long term career in some of these areas, if you find the right network there are a lot of opportunities to learn on the job and develop a career over time. If you're interested in larger-scale growing, there are likely farm internships that might be worth pursuing too.
Good luck!