You cannot use BaseCamp or other software to 'trim' an already compiled IMG file. You can load the map data to your PC via GarminExpress and use MapInstall to send a reduced size IMG to an SD card, unlock it with gimgunlock and then transfer it to your 205. However with latest firmware loaded, your nuvi 205 can see IMG files of up to 4GB (with old fw the size limit was 2GB). So just use a 4GB SD card for your 205 and it can use the entire North American map. Best not to put unofficial unlocked maps in it's internal memory anyway, if the IMG is corrupt it could 'brick' the nuvi . On a media card it doesn't matter, remove the card and the device behaves normally. Post again if you need more info.

BTW, because nuvi 205 doesn't have additional protection over GMA an older version of the unlocker should have worked unless the one you found was the very first one limited to the ability to unlock IMGs up to 2GB in which case it wouldn't have worked at all with an IMG over 2GB. It's more likely your original attempt was slightly corrupted in some minor way, check the IMG in GMapTool ('Details' or 'Info' tabs) if you can be bothered now:
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