It gets only the ip address of the default-gateway via DHCP. For sending packets to its default-gateway, it needs in addition to the ip address also the MAC address (at least in ethernet networks). ARP is the protocol to get the corresponding MAC address for a known ip address, so the host uses ARP to get this mapping (if that mapping is not already known by received packets from the gateway).
(edited to correct some typing errors ...)
Host knows IP address of Gateway but it doesn't know MAC address.
"The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given internet layer address, typically an IPv4 address. This mapping is a critical function in the Internet protocol suite."