I'm assuming this is retail, here.

If it's on the schedule, and it's posted before you actually have to work, then it is in stone. If you don't show up for the shift, that is considered a 'no call-no show' and most companies have the right to fire you for that. Depending on the management, they can let you go, if they have a 'no call-no show' policy. It's technically job abandonment.

The idea behind it is, the schedule change has been made and posted before the actual shift starts, and you have had an opportunity to check the schedule and note the change. Which you have, so management has done it's job. Regardless of whether you made plans on the day you thought you had off, you could have either requested the day off after you made the plans, let your manager know you couldn't work that day after you found out (a note on his/her desk would have done), or found your own coverage. You did none of that.

Also, the no gas money excuse doesn't work. Most times, cities have buses. And, if transportation becomes a problem you could lose your job over that too, so I wouldn't bring that up.

Bottom line. Go to work.