Well, the e-commerce theme oftentimes isn’t the solution – if you are considering a theme, pay attention to what it offers
Most of the time, an e-commerce theme will simply be a theme the author feels is suitable for an e-commerce site. i.e. it looks good for a shop, rarely do they actually have the shop functionality built in. They may be compatible with certain e-commerce plugins
The e-commerce plugin gets you the store functionality. There are many like eshop (the one I”ve used most), jigoshop, wp e-commerce, etc. You really need to google around and do a little research on these, different people will rate these plugins very differently depending on their needs.
The payment gateway is the way you accept payment. Some e-commerce plugins may have them built in. But to take money, you need someone to handle the transaction – especially in the case of a credit card or something. Paypal would be a gateway for instance. There are many of these as well
I’d suggest reading around the web for wordpress e-commerce and see what you can learn.
If you are just offering the ability to pay online for one thing, you may not need a full fledged ecommerce plugin, especially if you aren’t selling multiple products, or stocking inventory, etc. just a way to take payment and provide a receipt/ticket.