If your gossiper crashes mid-download, you should have the file indexed (so it is available for search) and know which chunks are present when the gossiper restarts. Essentially, this means that you should index the file and save the information about it in a persistent storage as soon as you obtain its metafile (because at this stage you have all the necessary knowledge). That being said, I do not think that we have any tests that specifically check this behavior.
In terms of download itself, you do not need to resume it after the crash. Assume that it is a one-time command.
Let me know if anything is still unclear.