In a similar situation, I have used nail plugs. Choose the length so that the expanding part ends up in the Leca. You can screw them if you don't want to nail. Just be careful not to drill deeper than the plug's length, otherwise the "collar" on the plug might pass through the gypsum hole if you're unlucky.
If you plan to hang heavy items, I would also, if possible, try to make some attachments to the metal studs, and supplement with some drywall anchors here and there.