What does a subnet mask do? Almost nothing. In case of Ethernet:
— If some data is to be delivered to an IP address that belongs to a subnet, send a packet with that destination IP in a frame with corresponding destination MAC. If you don't know that MAC, use ARP to learn it.
— If some data is to be delivered to an IP address that does not belong to a subnet, send a packet with that destination IP in a frame with gateway destination MAC. What happens next is not our problem. If you don't know that MAC, use ARP to learn it.
Therefore, /32 simply makes host relay packets to all other addresses to gateway.
Note that gateway IP address is not included in any packet. We only use it to learn gateway MAC address we actually need. Note that gateway IP address can be anything, inside or outside of local subnet. The only requirement is that host gets an ARP response when asking about it (from actual interface or some other device acting on gateway's behalf). This gets trickier when host have multiple interfaces to multiple networks, and may have to prioritize, or some interfaces depend on others (as with VPN connections), but it's certainly not a problem in case of a single network connection. Note that physical network connection does not imply that only packets from a single subnet get transferred. Anything can be sent or received, the difference is in what gets accepted and what gets rejected.
— If some data is to be delivered to an IP address that belongs to a subnet, send a packet with that destination IP in a frame with corresponding destination MAC. If you don't know that MAC, use ARP to learn it.
— If some data is to be delivered to an IP address that does not belong to a subnet, send a packet with that destination IP in a frame with gateway destination MAC. What happens next is not our problem. If you don't know that MAC, use ARP to learn it.
Therefore, /32 simply makes host relay packets to all other addresses to gateway.
Note that gateway IP address is not included in any packet. We only use it to learn gateway MAC address we actually need. Note that gateway IP address can be anything, inside or outside of local subnet. The only requirement is that host gets an ARP response when asking about it (from actual interface or some other device acting on gateway's behalf). This gets trickier when host have multiple interfaces to multiple networks, and may have to prioritize, or some interfaces depend on others (as with VPN connections), but it's certainly not a problem in case of a single network connection. Note that physical network connection does not imply that only packets from a single subnet get transferred. Anything can be sent or received, the difference is in what gets accepted and what gets rejected.