If you see X2-AP handover request message. Then GUMMEI is a mandatory field. As part of X2 based handover procedure, target enodeb need to send Path switch request message to MME. To send this message to relevant MME, GUMMEI value is required since MME UE SIAP ID is not sufficient to send "Path Switch Request" message to correct MME.
I think I have answered your query. Feel free to ask any further query.