The effort will be huge, unless you can somehow change the inner workings of Moodle's get_string function to use gettext internally without any other part of the code being aware of the fact. I don't know enough about gettext to comment on that.
Actually, it seems that you don't want Moodle to use gettext for its string handing. You just want to be able to use gettext's associated translation tools. That opens up another possibility: Could you write a script that converts moodle language files to gettext format files and back again. If so, you can use gettext's tools to do the translation without having to change Moodle.
Languages
using gettext
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