I have downloaded a couple .img files and used GMapTool to split to install in Basecamp / Homeport. BlueChart G2 V13.50 only shows 3 boxes in Homeport.
Bluecharts are made to be displayed in Homeport,no need GmapTool, copy it in an sd in a Garmin folder and plug it in your pc.
Gmaptool can be used to see BCs in Mapsource, but they will loose many functionalities, the only reason to do it can be the option of map printing.

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all these are in
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For Bluecharts use Homeport