Hi Asterios76,
Before using the po/mo files provided by Gravity Forms you need to find a way to use the same form across the sites in the Network.
So let′s say that you have a post in site 1 with this form:
[gravityform id=”1″]
Then you connect this post to site 2 using MultilingualPress so you have the same shortcode in the post, the problem here is that the form with id 1 does not exists in site 2.
As far as I know, by default is not possible to share a form across sites in Gravity Form, but maybe there is an extension (or some code snippet) that allows you to do it.
So in resume, once you are able to share the same form across sites in the network you can then look at translation of form fields (using po/mo provided by Gravity Forms).
Thanks,
Emili