There's no procedure to do direct mobility between GERAN/UTRAN and 5GS according to spec, and IP preservation is not supported either through indirect mobility GERAN/UTRAN<->EUTRAN<->5GS.
Regarding 3GPP TS 23.502 Annex G, open5gs-smfd supports Gn interface but only when working against a 4G MME, not through 5GS (PCF and AMF).
Related specs: *3GPP TS 23.501: ** 5.17.2.4 Mobility between 5GS and GERAN/UTRAN ** Annex L (normative): Support of GERAN/UTRAN access * 3GPP TS 23.502 4.11.5 Impacts to 5GC Procedures * 3GPP TS 23.632 5.3 5GC-EPC Mobility Scenarios
