make my gantt chart show every day of the month.

Unless I am misunderstanding something:

1) Go into the format axis dialog for the horizontal axis. You will see that the major and minor units for the axis are set to automatically and Excel has chosen 5 days for the major unit and 1 day for the minor unit. You can set these values to whatever you want.
1a) To get an unlabeled tick mark at every day, leave the major unit and minor unit at 5 and 1 and format the axis to show minor tick marks.
1b) To get a labeled tick mark at every day, change the major unit to 1.
1c) To add minor gridlines, go to Chart tools -> Layout -> Gridlines -> Vertical gridlines -> show major and minor gridlines.
1d) Whether you choose major or minor units/gridlines to represent 1 day, be sure to uncheck auto so that Excel will always leave this value as 1.

Is that what you are trying to do?