Can I use a rangefinder or GPS device?

Under the Rules of Golf, yes: getting distance or direction information from a distance-measuring device or compass is allowed (Rule 4.3a(1)).

Not allowed during the round:

• Measuring elevation changes (turn off any slope function)
• Using a device that interprets information, such as recommending a club or line of play based on your ball's location
• Using an alignment device to help aim

The Committee can ban distance-measuring devices with a Local Rule (Model Local Rule G-5), so check the terms of the competition.

Penalty for misusing equipment (Rule 4.3): general penalty for the first breach (two strokes in stroke play), and disqualification for a second breach.