[Question] Does "You are legal to stay" mean I don't have a gap in my legal status?
-Correct.
My H1 I-94 card wrote 07/13/2012 now but my H1 will be withdrawn by my company on 11/19/2010, will this change the I-94 expiring date to 11/19/2010?
-Not affect the I-94 duration, but you are out of status after you lost the job unless you chnage your status or do h1 transfer.
Sorry that I am confused with the concepts of "legal to stay" and "unlawful presence", do they mean the same?
-No. Legal stay means you can stay US legally at a certain period of time though you do not have a status e.g. you can leaglly stay until your I-94 expires even if you are laid off (you are out of status) and USCIS/CBP will not bother you and not put you in removal proceeding.
Unlawful presence means you are illegal to stay and USCIS or CBP may put you in removal proceeding and ask you to leave US if they find you illegally stay.
Anyway, you need to find a job as soon as possible or change your status to another non-immigration status such as H4, F1, B2 etc. after you are laid off.