You still need a baseline average of your typical distances to go by, and GPS helps you learn your yardages so you know what club to pull in the first place.
You "guess" at all of these already. GPS just gives you an accurate starting point to work from.
How can you know for sure that you didn't pull the wrong club?
This is probably the most important point: distances markers on golf courses aren't always accurate and even if they are, they only give you the distance to the center of the green. They don't tell you how deep or wide the green is, what distance you need to avoid hazards, etc. They also don't help you off the tee, either.
If knowing the distance isn't important to you, then why would you ever choose an 8 iron for a shot instead of a 7 iron?
But, if all the courses you play have plenty of easy to locate distance markers, then you probably don't need to spend a fortune on a range finder.
Here's a free one: https://play.google.com/store/apps/details?id=com.mozosoft.rangefinder&hl=en
As of late I have played with a few people using some sort of GPS device, which in turn made me envious, and I have therefore been looking on the internet with a view to buying one. Then I stopped and thought, WHAT DO I NEED IT FOR. Firstly, I am not a great golfer, and the length I can hit a ball with a club varies a lot based on how well I make contact. Secondly, what if it is windy, what if it is cold, what if it is wet, and finally, what if I am hitting a ball uphill or downhill. All a GPS would help me do is estimate what club I need on a dry day, with no wind, and on a flat plain. Of course, I and can take these factors into account, in which case I would be guessing again, and then I may as well not have one anyway. Like most people, I quite often hit over the green, or leave it short, but to be honest that has more to do with my golfing skills rather than the wrong club. All courses in the UK have distance markers, and to be honest you have to be pretty stupid not to be able to work things out from there.
Needless to say, some of you would have already spent a fortune on such a device, and will reply to this post telling me how wonderful they are. So, please go ahead and convince me !!