Locally. The network, broadcast and gateway addresses do not exist. They are artifacts for your routing table. The modem will actually forward traffic for all 3 to the ISP, so that you can reach other customers being assigned those addresses from the pool (if it weren’t for the firewall rules which probably forbid this).
The modem gets one address from the ISP. It calculates a mimimum netmask alllowing a virtual gateway address in the same subnet. The size of the subnet depends on how many of the least significant address bits are zero. The requirement is that “address & netmask != address”. The netmask must also allow one other address, for the virtual gateway, so the absolute longest mask possible is 255.255.255.252
You got 10.7.8.45. The LSB is set, so the modem can use the long mask. The gateway is then the only other possible address in that subnet.
If your ISP had assigned e.g. 10.7.8.64 to you instead, then the modem would have had to use a mask with 7 zero bits. I.e. 255.255.255.128. It could then use any of the 125 other addresses as gateway. I don’t know which one it will choose.