But what could "Glowgie" be?
There is a village called GLEWO situated on the opposite side of the Vistula river east of Włocławek.
There is avillage called WIELGIE north-east of Glewo. This one suits your Glowgie better, in my view.
I see from old maps from the 1920s that there is a Dąb Wielke and Dąb Mały right in this region
There are three of them now: Dąb Wielki, Dąb Mały and Dąb Polski.
Her brother's naturalization papers say they lived in Dembe before coming to the U.S
The Polish spelling for that would be DĘBE. This spelling "Dembe" is important as it preserves the change of the letter a into the letter ę in some Polish declinations, like in dąb --> dębowy. A person who lived in the village of Dąb would say: "Mieszkałem w Dąbiu" rather than "Mieszkałem w Dębem".
I wonder if by saying Grossdorf she could have meant the "big" Dąb in a manner of speaking
She could have well meant "Wielgie" as well since "Wielgie" is a variation of "Wielkie" in standard Polish which means "gross" in German.