GlaDOS is a psychopathic monster, and always intends to kill you. If you don't escape the incinerator, or you fail to shut her down during the neurotoxin sequence, you die. She never has any intent to give you the cake or the party - these are just things she uses to entice you.

At the end, you can see that she could have made good on her promises, even though there is no outcome in the game where she does.

With regards to the cube, however:

She really does have your cube, and she returns it to you in Portal 2 when she ejects you from the testing facility.

GLaDOS is a psychopathic monster, and always intends to kill you. If you don't

escape the incinerator, or you fail to shut her down during the neurotoxin sequence, you die. She never has any intent to give you the cake or the party - these are just things she uses to entice you.

At the end, you can see that she could have made good on her promises, even though there is no outcome in the game where she does.

With regards to the cube, however (Portal 2 spoiler):

She really does have your cube, and she returns it to you in Portal 2 when she ejects you from the testing facility.