Since you are still employed by your previous firm during garden leave you are still covered by your employer’s health insurance policy. I’m also on garden leave at the moment and that’s how it works in my case.

I’ve had this where it’s covered, but also had it where it’s not covered.

There is the option to COBRA. I just switched to be on my wife's insurance.

In my (limited) experience, COBRA is far, far more expensive than buying on healthcare.gov.

COBRA just means that you’re continuing whatever plan you had under your employer, at full cost to you (vs. the employer presumably subsidizing some of the cost while you’re employed by them). The amount you pay is entirely dependent on the plan(s) your employer has chosen to provide its employees.

For me, the full cost of my current employer’s health plan is about $1800/month, and a comparable plan from the healthcare exchange is $2200-3000, depending on the plan. So if I were to lose my job, it’d be significantly cheaper for me to use COBRA than get a plan from the exchange.