The squeak is most likely to be coming from the point of contact between the clutch fork and the thrust bearing.
I have seen this many times on subarus.
It can usually be lubricated to stop the squeak without removing the gearbox.
It is a careful and delicate procedure, because you have to be careful not to lubricate the clutch friction surface in the process.
The other place that the squeak can come from is where the clutch fork is pushed by the rod coming from the clutch slave cylinder.
This is much easier to lubricate.
Try this first.