If I were you, I would answer "yes" and explain the details in a separate page. The reason I am recommending you to answer "yes" is that it shouldn't make you ineligible to apply for GC. If you had applied for food stamp or similar, then that would be a separate story.
On the other hand, if you answer "no" and if an immigration officer thinks you answered untruthfully, you might run into problems. If you have some sort of documents about that medical assistance, attach them as well with your I-485 application.

Disclaimer: I amn't a lawyer, and please take this suggestion with grain of salt, of course.