Everything Marwan said is correct, but let me state it in a diffrerent way to clarify.

Trunk: A trunk refers to any communication line that carries multiple 'channels'.

In the data (IP) world, a trunk connection carries multiple virtual LANs (VLANs). For example, in VoIP, a trunked IP link will often carry a Data VLAN and a Voice VLAN.

In the voice world, a trunk usually refers to a channelized T-1 or E-1 TDM (i.e. non-IP) connection. These are usually PSTN connection provided by the Telco or a connection to a TDM-based PBX. However, a voice trunk can also refer to a SIP trunk or an inter-cluster trunk. These are both IP-based trunks, and are therefore virtual, but otherwise act like TDM trunks.

Gateway: This can mean so many things that it doesn't mean anything out of context.

A data gateway is merely the next-hop address for IP data packets. It is what is configured on end devices that allow they to route packets outside the immeidate network.

A voice gateway (Often called a VG) is typically the point between traditional TDM, channel-based voice networks like the PSTN and newer packetized, IP-based voice networks. These devices are responsible for converting digital or analog channels into IP streams and converting IP streams into digital or analog channels. They also must understand and convey address information, such as Called number and Calling number between these networks.

Therefore, the short answer to your question is that a gateway is (typically) a physical device, whereas the trunk is (typically) a cable connection to another device.