I have a query on how to compute round trip delay. I know it is 2 times the propagation time, but let's say I have a packet of 100 bits and a transmission speed of 2 Gbit/sec, how do I compute the round trip delay in this case?
You can't calculate it without knowing the delay between two hosts.
If the other host was on the moon, it takes about 1.3 seconds for light to reach it, 1.3 seconds to come back. Round trip time would be at least 2.6 sec. (A tiny bit more because it's not actually a vacuum, and plus 100/(2 x 109) sec for the packet. Plus any additional framing and packet overheads, plus far end processing time.)
In real life, network round trip time is dominated by switch and router transit times for shorter distances, speed of light through fibre for intercontinential, and speed of light through vacuum for satellite. For LANs it's often network interface and operating system detail.
Why is RTT hard to calculate? Round-trip time is a complex metric that has several components. It includes propagation delay, processing delay, queuing delay, and encoding delay. Propagation delay is usually the dominant component in RTT. It ranges from a few milliseconds to hundreds of milliseconds depending on whether the endpoints are separated by a few kilometers or by an entire ocean. The remaining components (processing, queuing, and encoding delays) can vary by the number of nodes in the network connecting endpoints. When only a few router hops separate endpoints, these factors are negligible.
So what can I do? Actually calculating this value is rather difficult, as mentioned by jonathanjo. However, the ping time may be a reasonable estimate for RTT. It may differ in that most ping tests are executed within the transport protocol using ICMP packets. In contrast, RTT is measured at the application layer and includes the additional processing delay produced by higher level protocols and applications (e.g. HTTPS).
Check out this article to learn more about round-trip time.