Commented by: SAUL HERNANDEZ (saul.hernandez)

Remember that GEN_ID use a secuence and this is a global object and you can get last id en everywhere.
I recommend that you think about Identity column in firebird , and use when you need know las id
the next statement:

INSERT INTO PERSON(NAME,LAST_NAME) VALUES('JOHN','MONTANA') RETURNING IDPERSONA

(With this you get the id generate in firebird in a secuence or with identity column)

AND if you want store the id in a variable you can use this statement

INSERT INTO PERSON(NAME,LAST_NAME) VALUES('JOHN','MONTANA') RETURNING IDPERSONA INTO :IDPERSONA (THIS IS A VARIABLE)

I DONT RECOMMEND GEN_ID(....,0) USE FOR GET LAS ID , BECAUSE OTHER USER CAN INSERT RECORD IN THE SAME TIME AND YOU SECUENCE MAYBE DONT BE EXACT