If I had a $ for every time someone has asked me that exact same question.
IMHO once you reach a certain price point many of the windows/doors become much of a muchness, all have Low E and low E doesn't change from manufacturer to manufacturer, all have argon, same argon for all, all are double, triple or quadruple welded vinyl most upmarket ones have steel reinforcing etc...etc...
Where I think the difference comes into play is if you want wood interior with vinyl exterior, customer service is another big thing though I will say even the cheaper end companies have picked up their game in this area.
Pella have a couple of lines, one for Lowes and then their upmarket Pella
Anderson are the same with Home Depot.
Marvin are exclusive to dealers (so only one line of product)
No Idea about Gilkey, though I'd be surprised if they weren't part of one of the bigger guys or a mass production company like Simonton.