How can I open a file that results from grep?

Use grep -l to just get the filename of the matching file and not the matching text, then combine it with vim:

vim "$(grep -l some_pattern file_names)"

You can use quickfix or errorfile feature in vim:

$ grep -n foo * > /tmp/foo.list
$ vim -q /tmp/foo.list

Vim will open the first file in /tmp/foo.list and place the cursor directly in the line where foo was found. You can go to the next instance using :cn and previous instance using :cp.

Side note: If you are already using vim or gvim, then I would suggest using its inbuilt grep functionality. Read :help vimgrep for more information.