Integrating a foreign GEDCOM file is one of the more difficult problems any genealogy program faces. Gramps is no different.
I would start by opening a new tree and importing the GEDCOM and looking it over for issues. If there are people that you don’t want, this is a good time to delete them. If you trust your own work for common people, you may want to delete the common people, at least those away from edges that may have new and interesting people. You don’t have to do this, but it does get you some familiarity with what is likely to need further work and will save you some effort later.
If you do this step, export your work as a Gramps XML file.
Backup your own tree in case you decide you need to start over.
Once you are ready to start integrating, import the foreign file (either the GEDCOM or XML, if you did that) into your own tree. Everyone and all the data is likely to be imported as new objects, even if they are common between the two trees.
Now comes the hard part, merging. It is largely a manual process, usually starting with people. One tool which can help a bit is the “Tools/Family Tree Processing/Find possible duplicate people”. This tool may help with matching up common people and performing the merge. Or you can just sort the persons by name and do it manually. After a merge, it is a good idea to open the person with the person editor and look at events, citations, etc. and see if duplicated data is there, and delete it or make a note to merge the item if there is good stuff in both almost alike objects. You can even go to the view containing the objects and perform the merge while you have a person edit open.
Then you can do the families, no tool for this.
If you delete objects like events, notes, media etc. from your families or persons along the way, you will end up with these ‘unused’ objects in the other views. You can use the “Tools/Family Tree Repair/Remove Unused Objects” to clean these out of your tree. You may have to run this multiple times to get them all, as removing an event may make a place unused etc.
As you can see you will end up doing a LOT of manual work, no real help for this at this time. Performing a merge automatically is going to require almost Artificial intelligence, and we are not there yet.
Make frequent backups, in case you need to blow off a big change. And remember, you can Undo changes if you make an error within the same Gramps session.