Gdansk to Malbork Castle... a day trip or an overnighter or an on the way day stop to Warsaw?

Do it as a day, or rather a half-day trip: it is - depending on the type of train - only a 30 to 45 min journey from Gdansk with frequent connections, and in Malbork a 15-min walk from the railway station.
For your travelling use the planner of the Polish national railways.

Gdansk has two sights that can be extraordinarily time-consuming--depending on your level of interest, of course. I spend more than double the typical amount of time at 20th-century historical sights, because I read every word of the posted English-language material and watch all the subtitled videos. The Poles, bless their hearts, have gone all-in on the English bit. I spent about 20 hours in the WWII Museum and 8 hours at the Solidarity Center. I also enjoyed the small amber museum, and of course the city was beautifully rebuilt after the war, so it's worth many hours of wandering around. I didn't have time to go to Malbork.

You should expect heavy tourist crowds in the historic district, and it would be a good idea to buy your ticket to the WWII Museum online before heading over there if you wish to see it. I got held up at the ticket counter for about 20 minutes on one of my visits in mid-summer 2018. I think the museum is fairly new, and I got the impression many of the visitors were Scandinavian tourists; I wonder whether it's busier on weekends.

Malbork works well as a stopover or as a day trip. When we were there in summer 2018, there were luggage lockers at the station (not a lot of lockers, but we had no troubles getting one about the time the castle opened) and there are many more lockers at the castle (downstairs from where we bought tickets). We visited on the day we traveled from Gdańsk to Toruń (we needed to change trains to get from Malbork to Torun).

If you do a daytrip from Gdańsk, you don’t have to mess around with luggage and you would have more flexibility on your return time. But if you were to stop in Malbork on your way from Warsaw to Gdańsk, you would “save” the travel time of a round trip Gdańsk/Malbork.

Thank you for everyone's great advice! After having read what you have to say I have planned to go to Malbork for the day as a stop on the way to Warsaw. It is good to know that there are luggage lockers at the train station in Malbork so I can have a nice time at the Castle. I do not need to be in Warsaw at a specific time and since distances aren't too great I think that I could have enough time at the Castle. I checked the train timetable that was provided in the comments and they trains seem to run quite frequently between Malbork and Warsaw so I can choose to catch a train later on in the afternoon or even early evening. Also this will give me more time to explore Gdansk and the surrounding areas. Thanks to acraven for the advice on visiting some of the sights of Gdansk as well. I am excited to see the solidarity sights and am looking forward to the WWII museum. I really appreciated your honest advice and cannot wait to use it on my journey through Poland! :)

Thank you OP for this question and all the helpful replies. I am planning 20 days in Poland this spring, and was about to post a very similar question. I'm trying to decide whether to stop in Malbork enroute from Warsaw to Gdansk. The travel day happens to be the Saturday between 2 Polish holidays - Labor Day and Constitution Day. Might it be better to wait and take it as a day trip on Mon or Tues? Or even as an enroute stop between Gdansk and Torun on Wednesday? We'd rather not have to fight crowds, if any, on a holiday weekend.

Thanxamillion!

I did it as a day trip from Gdansk and it is easily doable. First I was planning overnight because they have few hotel rooms right in the castle but they were all taken. In the evening they had some performances going on and fireworks. Of course I could sleep somewhere else in Malbork (town) but because I could not sleep at the castle I did it as a day trip.

I spent two nights in Malbork town and thought it was well worth it, in the morning, I had the castle to myself before the daytrippers from Gdansk arrived, and in the evening, I took a private night tour of the castle after the Gdansk daytrippers left, I'd compare it to staying the night in Toledo or day tripping there from Madrid.

Because I was staying the night in Malbork, it gave me a spring board to visit other further sites, like the Grunwald Battlefield (largest battle in Medieval Europe) or the Masurian Lakes District, which would have been too far if I was based out of Gdansk.