If you get a root canal, is it still possible to get gingivitis?

Yes: Both are totally unrelated and caused by different factors.

Yes: They are not connected. Root canal treatment is inside the tooth. Gingivitis is inflammation (redness & swelling) of the gums (around the tooth or teeth).

Yes,: Rct is a nerve issue with the tooth and gingivitis is usually an oral hygiene issue with the gums.

Different infections. Different bacteria. Different locations. Both harmful to both the oral tissues (hard and soft) and other organ systems. Sounds like you would be well served by following a Dentist's instructions to bring you mouth back into a state now health.