3GPP 23.501 v20.2.0 — the document's own text
8.3.2 User Plane for untrusted non-3GPP Access
Taught in 8. What actually travels on the wire (The 5G system architecture, in depth).

Large GRE packets may be fragmented by the "inner IP" layer.
Details about the PDU Layer, the N3 stack and the N9 stack are included in clause 8.3.1. The UDP protocol may be used below the IPsec layer to enable NAT traversal.