Based on the original team rocket I thought the team Rocket Grunts would be ekans, koffing and meowth. I have caught several of them and the counter for the research task hasn’t gone up.
I’ve not spotted anything that would show what one of these grunts might be. What is a Team Go Rocket Grunt?
The team rocket grunts are actually people, well in game people anyway. Keep an eye out for a pokestop acting strange. The blue cube on top will be dark and will be spinning around in a weird way. When you interact with it, a grunt will pop out and challenge you to a battle. Defeat the grunt in a pokemon battle and you will get +1 added to this research task challenge.