I wanted to install gb
The installation steps says to execute the command:
go get github.com/constabulary/gb/...
What does the ... mean in this case?
The ... (ellipsis) tells go get to also fetch the package's subpackages/dependencies.
From go help packages:
An import path is a pattern if it includes one or more "..." wildcards, each of which can match any string, including the empty string and strings containing slashes. Such a pattern expands to all package directories found in the GOPATH trees with names matching the patterns. As a special case, x/... matches x as well as x's subdirectories. For example, net/... expands to net and packages in its subdirectories.
For an example of how you'd use it, check out this answer.