Does the train go to Baltimore to penn station in Baltimore

No, the MARC train that goes through Gaithersburg is the Brunswick line, which goes from Martinsburg, WV and Frederick MD to Union Station in DC in the morning and the opposite direction in the evening. If you want to get to Baltimore, I think your best bet is to take the 201 bus (MTA) from one of the several stops in Gaithersburg to the BWI MARC station (about 1 hour ride) and pick up the MARC Penn line there to Penn Station (about 20 minute ride)

I have coworkers that take the metro to union from the silver spring or rockcville stations to union and the Penn line from there

MTA bus 201 is the best option, only 5 bucks

You could also transfer at Union but you be paying the fare twice. Bus maybe better

If you live in Gaithersburg, you will have to catch the Brunswick line Marc train to Union Station then catch the Penn line Marc train to Penn station. Keep in mind the Brunswick line only runs in the morning to Union station DC.