When adjusting a portal's gantry length or repositioning its masts, the masts and gantry are locked together, as shown below. So whenever you change gantry length, the mast positions are changed accordingly, and vice-versa.
However, it is possible to adjust the components independently, using the 'Override Gantry Length' checkbox.
Useful info: If you need to know how to create a portal first, see here.
Step 1: Navigate to the 'Structures' section of the Lineardraft toolbar
Step 2: Click the 'Adjust' button
This will prompt you to click a marker.
Step 3: Click on the portal structure you would like to edit
This will open the 'Edit Structure' window.
Step 4: Click the 'Settings' button
This will open the 'Settings' window.
Step 5: Tick the ‘Override Gantry Length' checkbox and click 'Confirm'
This will close the 'Settings' window.
Step 6: Adjust the gantry length or mast position(s)
You can now adjust the gantry length without changing the position of the masts.
You can also adjust the position of your masts without changing the gantry length.
Step 7: Press 'Confirm' to save your changes
This will close the 'Edit Structure' window.