Change gravity by touching a part using the code builder

Place the part to act as your button. Select it and open up the code builder.

From the Events, start with a When player touches object, followed by a change plot's gravity to -- part from the Game Section. Change the number to 15 or whatever number you'd like. The lower the number, the less gravity.

From the Control section, place a wait for -- seconds and change the number to however many sections you want the low gravity to last.

Lastly, place another change plot's gravity to -- part, to set the gravity back to normal.