Galaxy specific questions are best posted to their help forum: https://help.galaxyproject.org/

That said you probably may need the uncompress tool: https://usegalaxy.org/?tool_id=CONVERTER_gz_to_uncompressed&version=latest

Tar files provide no compression compared to the originals. The intent of tar utility was to group many files into one tape archive, which is subsequently compressed by gzip, bzip or a similar utility. My advice is to either upload the original files - should be no different in terms of size - or better yet to compress the original files directly with gzip. I assume Galaxy would work with .gz files.