Indeed there are many ways to do this with G’MIC. Some questions might narrow it down:
How accurate does it need to be (e.g. is this for a study/paper and needs to be recorded to avoid manipulation issues)?
Does the colour need to be matched, or only brightness?
Are you planning to do this using only the GUI, or command line?
As for suggestions… assuming accuracy isn’t a problem you could try blend [seamless] for merging. G’MIC also has a curves filter. Other than that the list is enormous!