The home gateway is the point at which all the L2TP tunnels are terminated - after that it's TCP/IP traffic.
Home gateways come in various traffic sizes - the most common being 34Mbps and 155Mbps. They also come in various "flavours" - ie you can have BT Wholesale provide and manage the physical hardware or you can just have them provide a data feed and your ISP provides and manages the hardware.
The 34Mbps gateways give the ISP no choice - the hardware is supplied and managed by BTwholesale. The 34Mbps gateways support a maximum of 1600 users, although "real-world" usage indicates that 1200 is a more sustainable figure.
The 155Mbps gateways give the ISP a choice of whether they want to provide and manage their own hardware. The product that allows them to provide and manage their own hardware is called L2TP Passthrough and supports a maximum of 8000 users - once again this is likely to prove unrealistic with some ISPs already talking about a maximum of 4000 users being the limit. The BT managed gateway supports a maximum of 6400 users.