Thanks to everyone for the comments.
So, I had a chance to measure and inspect the GSP CV shaft and I found the following:
Both right and left GSP axles are identical in length -- which I wanted to double check;
The OEM stub axle is the same length on the passenger side as the GSP axle measuring from the collar to the end of the chamfer end. I have not taken the driver side stub axle out yet but this will certainly be different;
The chamfer end of the GSP axle is very rough compared to the OEM stub axle -- see the two pictures below of the GSP axle. I also noted the splines at the end of the GSP axle do not appear to be as consistent in thickness. Specifically, the splines appear "fatter" near the chamfer end of the shaft. Maybe I am seeing things but you decide from the photos below;
I also measured the diameter of both the OEM stub axle and the the GSP axle and they are nearly identical at ~29.8mm.
That said, I wanted to see if, perhaps, the problem I was having seating the new axle was something wrong in the differential (damaged circlip, etc). So I removed the unseated axle and tried to replace the OEM stub axle into the diff. Long story short is that I was able to push the stub axle back in place with three or four firm hits with the base of my rubber mallet. Now, I would expect the new axles to be a bit tighter as they are new but I pushed and hit the new axles as hard as I could to no avail in seating them. I am not comfortable hitting them harder or loading the suspension with the axle unseated -- they may never come out.
So, the issue here is probably the roughness of the chamfer end and, I believe, the irregular or inconsistent spline thickness at the chamfer end.
At this point I intend to send the GSP axles back to RockAuto. While I could try to file and smooth the ends I am concerned that once I get them in I may never get them out short of a diff rebuild.
G-Tech I am interested in purchasing the OEM 280zxt axles you have for sale. Please tell me about the condition, price, and terms / shipping? Or point me to where I can find them and how to complete the transaction with you.