This has caught me out before, so this is as much for me as it is for you!
Scenario 1 - editor and gocode out-of-sync
If you write your Go code using a text editor which takes its Go auto-completion from gocode (it will), it may get out-of-sync with your editor every now and then, if your top-level Go package goplus for Atom or vscode-go for VSCode etc. doesn't update it for you.
Perhaps the most obvious symptom of this issue is the auto-complete dropdown. If you're only seeing
PANIC as an auto-complete option, run through the following steps to rectify:
gocode close go get -u github.com/nsf/gocode
You may need to close your editor for the changes to take effect but this seems to do the trick for me.
Scenario 2 - gb workspace layout
If you've configured gocode to work in gb mode using the following command and things aren't working:
$ gocode set package-lookup-mode gb
You have two options:
1 - roll forward and get it working in gb mode:
$ gb install all
2 - roll back to the default mode:
$ gocode set package-lookup-mode go