I am curious what is the difference between diameter of a graph vs longest path of a graph. I just read diameter of a graph can be solved using Floyd warshall in O(V^3) while longest path can be calculated in O(V + E) using topological sort.
The longest path of the graph is the longest path between any two vertices. There may be several with the same longest length.
Since you are comparing with the diameter, which is an integer, you probably mean the length of the longest path
The diameter is the length of the longest of the shortest path between any two vertices. That means that you compute the shortest path for any pair of vertices, and take the longest one of them.
The distance between two vertices being the shortest path, the diameter is the longest distance between two vertices,
In general, they are not the same thing. Also, for the general graph, it is easy to compute the diameter, but hard to compute the longest path. In the graph below, the diameter is 4. A path from $6$ to $2$ is highlighted, which is of length 4.
However, there is a longer (simple) path from $6$ to $2$ of length 5.
Topological sort is only defined for directed acyclic graphs so you can't use it to find longest paths in general directed graphs or undirected graphs. Finding the longest path in an undirected graph (in the whole graph or between two specific vertices) is NP-hard. I assume this holds for cyclic directed graphs, too.
Note that it's "unfair" to compare Floyd Warshall with an algorithm for finding the longest single path in a graph. Floyd Warshall computes $\Theta(n^2)$ different things (the length of the shortest path between every pair of vertices) so it's running time is, in a sense, $O(n)$ per answer.