There are two kinds of zoom: gantterAi™ zoom changes the timescale of the Gantt chart, and browser zoom makes the whole page bigger or smaller.
Change the Gantt timescale
Click Zoom In or Zoom Out in the toolbar, or use View > Zoom In / Zoom Out (Ctrl+K / Ctrl+M). This switches the chart between time periods, for example from months to weeks.
Make the whole page bigger or smaller
Use your browser's zoom:
Windows: Ctrl and + to zoom in, Ctrl and − to zoom out, Ctrl and 0 to reset.
Mac: Cmd and + to zoom in, Cmd and − to zoom out, Cmd and 0 to reset.
See your browser's help for more, for example zooming in Google Chrome.
On boards, use the zoom controls at the bottom right of the board instead.