You got an error message with the first one because the command is not proper.
Are you able to copy and paste. It's best that you do that.
adb shell mount -o remount,rw /dev/block/mmcblk0p5 /system/app
(This mounts the drive as read/write)
If that is successful then try to uninstall:
ADB uninstall /system/app/GingerBread.apk
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