The charge capacity is mostly irrelevant (the Ah spec). (Within some bounds of reason.) The CCA figure is what's important. I'm fairly sure the stock battery is more tha 40CCA. As I recall the Yuasa YTZ7S battery which comes in the 570 has 130CCA. As far as I know there's the same or very similar batteries in the previous bikes.
btw - Gel batteries are lead-acid
Gel is a subtype of lead-acid, that is to say. It's mostly just a difference in what kind of vessel or form the electrolyte is kept.
I don't really like lead-acid batteries. I do like LiFePO4 batteries though - they're MUCH lighter! And they have a much longer lifetime measured in charge cycles (thousands of cycles!). Meaning you don't have to replace the battery as often. And LiFEPO4 doesn't self-discharge, in other words they don't drop charge just sitting there. (There's always some parasitic drain in the system when the battery sits in a bike.) And they're safe (they're not the same kind of chemistry as what people usually call Li-Ion which is unstable and can burn or explode. LiFePO4 is stable, inert, and at least relatively non-toxic).
Re. charge capacity: I'm using a 4-cell LiFePO4 with 2.4Ah capacity and about 130CCA in the 570. It starts the bike just fine and has plenty of starts in it.
It just so happens that the charge capacity is the figure that's commonly used. With lead-acid batteries, the Ah figure is a decent proxy for the important figure, the CCA. But it's confusing in the end as other battery chemistries have very diffferent CCA-to-aH ratios.
A good type of LiFePO4 is EarthX. There are good cells in them (A123), there's a balance/protection "computer" circuit onboard, the case is waterproof, and it fits normal battery boxes.
I'd like it if there was a cheaper, smaller version available from them.
I've been using A123RC.com batteries. They're fine for the price - $45! - but I'll go with something a little better next time. Thinking of trying NoBS Batteries (good cells, good connector, balancing tap.)
Just to dump some technical details out there
If you want to play it safe, get a reputed brand's version of the stock lead-acid battery