According to a quick survey of videos of people introducing her, American and British pronunciations are roughly the same (Maxwell was the founder of an environmental nonprofit, a philanthropic job that often took her to conferences and TedTalks, and before the U.N.). These people, who I assume confirmed how to pronounce the name before saying it in front of others, say “Gee-lane,” with a hard g. For the second syllable, some hit the “–ane” pretty hard, as in “Gee-lane,” or “G’lane.” Others go with “-ahn,” as in “Gee-lahn”. It depends on regional accent (this guy sounds like he’s saying “Glen” when introducing her at a conference some years ago). But regardless of accent, the g is a hard one and the s is silent. “Gee-lane.”