Adding common MAVLink message to stream

Extremely new guy here. I inherited a drone. I’m not a drone guy. I’m working with Mission Planner to stream GPS data from a drone to my PC using PX4 radios. Currently the drone broadcasts a subset of MAVLink messages, including Message 24 (GPS_RAW_INT). I can setup Mission Planner to reroute these MAVLink messages over my network and decode them in a program I’ve written. I would like the drone to also broadcast Message 127 (GPS_RTK), 128 (GPS2_RTK), or 232 (GPS_INPUT). In other words, I want to get GPS time from the drone. Is it possible to command the drone through Mission Planner to add these messages to its existing list of messages being broadcast?

Take a look at the SET_MESSAGE_INTERVAL mavlink message.

And read: MAVLink Interface — Dev documentation

Is there a way to do this through the Mission Planner GUI?

yes, in the control-f screen, i have feeling its not going to give you what you want though, as you are asking for messages that are likerly not fillable
ie GPS_RTK would mean you have an rtk gps onboard