It prints to stdout so you can of course grep your "gh issue list" (or at least the 3 fields that it displays: number, title, labels), but since it silently truncates output to at most 100 records, this doesn't seem terribly useful to me.
It doesn't even print a message or use a nonzero return code to indicate the output is incomplete. What's the point of running "gh issue list | grep foo"? There's no way to distinguish between "there are no 'foo' issues" and "there's 250 'foo' issues (but they happen to not be at the top of the list)".
Code search seems to be out of scope for the CLI. (It arguably doesn't even do issue search -- just basic filtering on a couple predefined fields.) But that's no problem because I've got all my source code on my machine already! I can use grep, git-grep, or any other tool I want (ack/ag/rg are popular).
Wait you can't grep over your case base if you use this tool?
Have they at least improved their code search? Last few times I used it it was very limited.