It seems EM stands for "The European Magazine and London Review " and GM stands for "Gentleman's Magazine". Musgrave is an index so you would need to try to find the relevant entries in the magazines to see how much information was given.
The GM 375 entry adds little other than a mention of Camberwell:
Behind a paywall at www.newspapers.com/newspage/34423322/ there's an entry for the Public Advertiser digitised "On Saturday laft ,died at Camberwell, Mr. John Stacye, an] eminent tea-dealer, oT Gracechj^rch-ilreet." So it might be worth checking more newspapers.
Also in the London Gazette, 7 March 1786: The copartnership between George Joyce and John Stacye, of Fleet-street, and of Silver-street, Fleet-street, London, Tea-dealers, being by mutual Consent this Day dissolved; all Debts due to, or owing by the said Partnership will be settled by George Joyce, who continues to carry on the Business; as witness our Hands this 6th Day of March, 1786. Geo. Joyce. John Stacye.