I’ve finished my game but I don’t know how to make a google play key 
hi,
google can make one for you, really the first time it takes the key gdevelop uses to sign your apk.
You can make your own key and sign you apk then google will generate publish keys, but you will have to sign gdevelop generated apk with your key before updloading to google play
Take a look at this document I did sometime ago
https://drive.google.com/open?id=1Fp9BS2MiRrvT2riQW1wCHPiSacnJKG3F
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