Brendan,
You are 15. You could have soloed a glider at 14. You can have a glider pilot's certificate at 16 (no such thing as GPL, PPL, CPT, ATPL in the US, those are European terms. We have Certificates, even if we say License.)
The "two pilots apply for a 737 job" is a very much Euro-centric statement. It has nothing to do with the way you get into a 737 in the US. Be careful reading "I want to be an airline pilot" on the Internet. The way it works in the US is entirely different than Europe. Europeans get in the right seat of an A320 with 250 hours, in the US that's going to be multiple thousands of hours in both seats of smaller aircraft first.
Gliding is not "a cheap alternative to power flying," but it's it's fairly cheap per hour once you are off tow. Many places only charge for the first few hours of your flight; if you stay up four hours, you only pay for the first three.
If you can find a place that uses a winch to launch you can get in the air much, much cheaper.
Consider finding a Civil Air Patrol squadron. If you are willing to trade your time for discounted flying you can have some great learning experiences. I am not an airline pilot, but I suspect the minimal regimentation in the CAP would be an easy way to ease into the disciplined life required to be a professional pilot.
The answer claiming that none of the gliding experience counts is completely wrong; follow the link there or read below.
How much of your gliding experience would "count?" Two parts to answer that. One, of the "minimum 40 hours" required for a Private Pilot's Certificate, only about half of it has to be in a "single-engine airplane." The balance can be in any aircraft (Words have meaning - a glider is an aircraft, but not an airplane.) Two, it may not all "count" but by the time you get your Private Certificate, you will have had numerous opportunities to grow, gain experience, and demonstrate responsibility - like taking passengers for rides - that you'd never get so early if you wait for a power plane!
If you show up for your first lesson in a Cessna 152 with 50 hours of glider time in your log book, having already flown higher and longer than a 152-driver can dream of, you will be well on your way to being a great pilot.
The Civil Air Patrol, by the way, has national glider camps where you can get as many as 30 flights in a week if you earn your way to acceptance into the program. When ready, as a Cadet you can even get instruction in power planes.
You've seen those VISA ads on TV? How much is a text book? $50. A parachute jump $150. Showing up for your Private Pilot Certificate Practical test with dozens of hours of real Pilot In Command time? P R I C E L E S S :)