Without seeing both, the source file and the printed result it is hard to tell, but I think you actually did some rookie mistakes. I am commenting those because those are the ones you can work with.
You do not send a 7Gb file to be printed... ever.
No one will work on that file seriously unless you are working in a really closed controlled environment, probably doing something for a Hollywood Blockbuster.
48x120 at 300 PPI is a file of 14,400 x 36,000 px or 518 Megapixels.
Considering that a really high-end camera has around 100Mpx, and a normal camera has 24Mpx...
An image of about 9 times smaller, 48x120 at 100ppi would work better.
Why is this important? because the person doing the job just resampled the humongous file to whatever he felt right. And somehow this is normal.
Are you a bad designer? I don't know about the design, but the file was prepared and sent in a wrong way. Again, that is a rookie mistake.
Sayed that, the print company personnel are just plain incompetent for two reasons.
If the file is not sent in a proper way by the client, they just need to inform you so you correct it.
If the file has enough information which it seems in this case, they just need to resample the file, for example reducing the file to 100ppi. No big deal there. But I still think point 1 is the proper way to act.