One way to do this simply in Excel, use as many columns per month as you need to get the divisions you want for project starts (2 columns/months would allow you to start a project at the begining of the month or halfway through, 4 columns give begining, 1/4, 1/2, 3/4 starts, etc.). Then in the row with the month headers, select the cell with the month name and the adjacent cells that are included in the month (select the month name and 3 adjacent cells if you are dividing the month into 4 columns). Use Format->Cells... and under the alignment tab select "center across selection" in the "Text alignment Horizontal:" dropdown.
Add borders to the appropriate cells to make it look good-I would add vertical borders to make one block for each month, with no visible lines delineating the columns within one month.
To get the bars below the month labels to represent the project duration, just select the corresponding cells and change their color (paint bucket button on toolbar or Format->Cells... "Patterns" tab.
Not as good as MS Project for planning, but it does suffice for making Gantt charts.