"TL;DR An excellently made Training rifle that is the closest you will get to replicating real firearm functionality through an AEG. Some minor qwerks pull it down slidghty, but if you are looking for something that has a bit more realism than what a PTW can do, while also looking for something that is less of a hasle than a GBBR, accept no alternatives.
*Disclaimer*, while I do own the GBLS DAS Limited Edition model, I do not own the GDR- 15, so I cannot comment on personal experience with certain external factors, such as the Centurion rail or the PTS butt stock, but the internals and over all functionality of the two versions are identical.
First things first, this is a product being marketed as a TRAINING RIFLE, not a normal AEG or GBBR that most airsofters would buy, so if you are just looking for raw performance and do not care about immitating the fuctionality of an AR-15, then this is not for you.
More importantly, the main function that makes this better than most other training platforms is that fact that it is an AEG that functions like a GBBR. It has a proper bolt carrier group that cycles each time you pull the trigger, while running off of a 11.1v lipo. This in turn also allows the rifle to have proper bolt lock back, so instead of something like the TM MWS where the trigger will not fire through electronics, the GDR15 will lock back the bolt on the last shot fired and will require the user to properly use the bolt release to charge the rifle on a new magazine.
Now with that out of the way, externally the rifle is suberb. Cerakoted and CNC'd steel lower and upper reciever with a PTS pistol grip that allows for a very comfortable posture when holding the rifle. The Rail is MLOK, and the barrel is close to 14.5" in overall length. Charging handle is a standard afair, with it being capable of charging the rifle, but not extrodinary. If you really want to change it, you could pick up a strike industries 5.56 ar-15 charging handle if you want an extended latch to grab onto. That leads into my next point, a majority of the parts externally will accept Real Steel firearm parts, with the only exceptions, that I have found, being the trigger, mag release button, and the fire selector switch. Speaking of trigger, it has around a 5-6 lb pull with a nice wall to break against (It does have a very small amount of take up, but you hit the wall almost immediately). The magazines are proprietary, while claiming to load up to 60 rounds, which you can do, I typically load up to around 45, so you will need to carry quite a few if you want to have a solid combat load of bb's available. The mags themselves are steel and have held up to me throwing them at walls and dropping them on concrete without losing the ability for them to feed. The price of them is a bit hard to swallow, but you are atleast getting decent quality. (Note: if you order from GBLS USA's website, they will throw in 5 extra mags, don't know if evike does the same or not).
Internally, the gearbox is proprietary, so parts wise you are kinda stuck with what you get. The good news is that what you get is of high quality. MIM and CNC Steel gears, CNC Steel trigger, CNC Sears, Everything inside of the gearbox shell itself is CNC'd, with said gearbox being CNC'd AL7075-T6 Aluminum, which is a rather nice metal for the shell. Due to the design of the gearbox, it functions in a split gearbox design, with the gears, motor, and sears being in the lower, and the bolt carrier group containing the spring, nozzle, and piston.The Bolt is charged when the user pulls back on the charging handle, which in turn puts tension on the spring and allow for the rifle to fire. This also allows GBLS to create multiple BCG to fit various FPS limits, however they (at the time of writing this review), do not sell sperate bolts. The inner barrel is a prommy 6.03 808mm barrel and the rubber is a standard AEG bucking, with a hop-up chamber looking akin to one of GHK's M4 GBBRs. I have not had a chance to mess around with the internals of the chamber itself, so YMMV.
Performance of the replica is solid. With .28s (bio/non bio), I have been able to hit targets at 35 yards away, so comparable to most standard hop-up/bucking combos in recently released high end AEGs. The actual realism functions is nothing short of glorious. Recoil is comparable to a low grain 9mm and the bolt being released makes quite a satisfying sound, which I have heard has only been improved on the GDR 15.
The main things that you need to look out for, along with some of my personal gripes, is that due to the stress of the bolt cycling through the rifle, it will be worth while to go through and apply some red loctite to the mag release button screw, optic mounts, and any thing else that you are going to attach to the rifle. Also, while the platform will accept real steel parts, you will most likely need to do some work to get it to fit properly (this occured to me while attempting to get a geisele mk8 rail to fit on, had to take a dremmel to the locking tabs to get it to slide on). Also, the hop up is a pain to adjust, as you need to use a long allen key, ala KWA style, to adjust a hex screw inside of the upper reciever towards the barrel. Speaking of the upper, make sure that you grease the Bolt with proper firearm grease, as the friction that it creates WILL blow fuzes if you don't lube it properly. So if you do get it, take some time to lube it and shoot it and repeat that for like 15-30 minutes.
All in all, a very exciting product that hopefully allows for more innovation into the airsoft scene. While the gripes I listed to most might be minor, you are paying quite a pretty penny. For those of you that are interesting in this training platform, and are willing to pay the premimum, my only advise would be to get a number of extra mags (I started with 6 and bumped up to 10), so that you can have a decent combat load, as I found myself switching to my pistol quite frequently due to either running empty at horrid times, or just being in prolonged firefights and running dry. Defeniately not for everyone, but a fantastic experience for those who decide to run it.