Gem au3 installation problem in custom path

I am trying to install au3 gem like:
gem install -i ./. --local c:/installs/au3.gem

it keeps on complaining “could not find a valid gem win32api in any
repository”. though I have installed win32api in the same directory and
in ruby installation path also.

au3 does not complain about anything when I do not use custom
installation switch ‘-i’ and it installs in Ruby gem directory.

now question is how to install au3 in custom directory.