First, a language matter:"Hannover" is simply German for "Hanover", just like "Deutschland" is German for "Germany".So you're actually referring to HANOVER.
Just like the U.S., Germany, too, has always been made up of STATES.According to what you say here anyway, these people were from somewhere in the former German STATE of Hanover.When talking about Hanover, we're talking about a state covering an area about the same as that covered by the U.S. states of Maryland and Delaware combined.So you won't be able to proceed until you do further research and come up with a record or document that names the exact city, town, or village in Hanover the Gill family was from.
It's always important to cite your sources.Where did you get the information from that this family was from somewhere in the state of Hanover to begin with?
Nobody here will be able to assist you at all anyway unless you provide a lot more information about these people.I can tell you right now that you won't get a reply to this or to any other query you might post if you provide such meager information.You're going to have to elaborate quite a bit if replies are what you're hoping for:
Did the specific people you mention here in your query emigrate to America, or did they stay in Germany?If they emigrated to America, when?And where in the U.S. did they settle?If they stayed in Germany, who in the family was then the immigrant, and when did that person come to America, and where in the U.S. did that person settle?Whom did your great-great-grandmother Margaretha marry, and when and where?As so many Americans for whatever reason do, you left out a piece of information that is always of the most vital importance in German genealogy: What was the Gill family's religious denomination?And last but not least, you say simply that you're looking for "information on".Well, "information on" is much too vague for anybody to be able to reply to your query.You will have to be more specific as to what it is you want to know exactly.