@GermanStar
Try this then:
Spoiler:
Open the GCD file in RGN_Tool and press the 'RGN" button saving the file as"XXXX01000xxx.rgn" where 'XXXX' represents the HWID of your nuvi 57 (should be 2087) and 'xxx' is the firmware version (e.g. version 4.50 is '450').

[Only registered and activated users can see links. ]

Drag the saved RGN file onto Updater.exe and enable the radio button next to 'USB Device'. Put the 57 into preboot, click 'OK' to execute the downgrade. Click thru any message about lower version.
Code:
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