=LOWER("g4a"&MID(CONCATENATE(A1,B1),RANDBETWEEN(1,LEN(CONCATENATE(A1,B1))),1)&MID(CONCATENATE(A1,B1),RANDBETWEEN(1,LEN(CONCATENATE(A1,B1))),1)&MID(CONCATENATE(A1,B1),RANDBETWEEN(1,LEN(CONCATENATE(A1,B1))),1)&MID(CONCATENATE(A1,B1),RANDBETWEEN(1,LEN(CONCATENATE(A1,B1))),1)&MID(CONCATENATE(A1,B1),RANDBETWEEN(1,LEN(CONCATENATE(A1,B1))),1))

Assuming the first name is in A1 and last name is in B1