GPD...What does it stand for?

[Delphinus]

I must have a 50gpd unit then. I repeat my earlier question, how do I tell what membrane I have? Is there some kind of rule of thumb I can use, i.e., length or width of the RO membrane? (It's just some Kent RO unit. Note it's not RO/DI, I don't know if that changes things. Paid maybe $75 for it. I didn't think I'd get such a good deal that it'd be a 50gpd unit for that kind of coin.)