Hi pafurijaz, it's difficult to thicken an open surface that is not all smooth because the offsets of each surface do not naturally touch each other and need to be extended and intersected with each other. The geometry library that MoI currently uses is not good at handling this.

if you use Edit > Separate to break it into individual surfaces then each of those can be thickened using the Offset command. They won't form a seamless result though.

Probably your best bet for making a thickened result would be to model the offset of the Gyroid using the same kind of technique you used for constructing the first surface.

Otherwise to get it to offset in MoI would require the surfaces to be smooth to each other instead of having creases.

- Michael