How to get the gain for this MOSFET amplifier?
Vgs = 5V so RsOn is low and you have a SOURCE follower ( or common drain ) with some unknown load resistance say 100x RdsOn which is missing between Source and Vss.

So it's a drain follower, like a unity gain emitter follower with a gain=&lt;1 depending on load regulation, Roin and a DC bias above Vgs(th).

But a rather sloppy logic diagram with errors... tisk.

Other
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Here is how I would make a 1GHz buffer Av =1 almost unity

I added Ciss and Rg to the model with very low RdsON Vt=1V and due to load regulation from Ron and insufficient Vgs bias from Vss the Vout DC is a bit more than -1V.



Go back and review your  common drain notes for details.