You're not going to get a gateway address inside the allocated /56 or whatever IPv6 block that's assigned to you and routed to your premises. If you somehow do, you politely ask the ISP to put someone on the phone who knows what they're doing. Or perhaps less than politely.
Usually, you don't have to worry about the upstream IPv6 address at all, as it will be autoconfigured as soon as you plug in your router. Most ISPs seem to be doing this with DHCPv6 (with prefix delegation), though it could also be done with straight up SLAAC if you have a statically assigned prefix.
Once the /56 comes into your network, you can subnet it however you like.
An example, with one possible (only partially fleshed out, and probably not very useful as-is) network design appears below. In any case, the IPv6 address of your upstream connection to your ISP is provided by the ISP and is outside your assigned prefix. You generally only need to worry about the inside interfaces. This example supposes you have an edge router with an integrated 4-port switch, such as many small business or SOHO routers.
IPv6 Address: (static, DHCPv6) IPv6 address: (SLAAC)
Gateway: (static, DHCPv6) Gateway: (SLAAC)
| Edge Router |
|-------------------| Your Prefix: __
| \ / | 2001:db8:3481:a700::/56
| X | ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
| / \ | Switch Ports example:
| VLAN trunk/switch | 1: VLAN 1-64 2001:db8:3481:a700::/58
+---------+---------+ 2: VLAN 65-128 2001:db8:3481:a740::/58
|ether2 3: VLAN 129-192 2001:db8:3481:a780::/58
+----> Core 4: VLAN 193-256 2001:db8:3481:a7c0::/58
Downstream in your core, you can further subnet these at core routers (or even layer 3 switches, more SOHO routers, etc.). I've also assumed every /64 will be on its own VLAN, though whether you do that is another detail you'll have to work out on your own.
You can also use DHCPv6 with prefix delegation to actually handle the addressing, which is probably easiest. Or you can go with SLAAC, which requires a bit more setup, or even assign manually, which I wouldn't generally recommend simply because it would be too labor intensive.