Make sure your spreadsheet file is uploaded to somewhere Geckoboard can access, with permissions set to be visible to 'Anybody with the link'.
See steps for these popular cloud services:
Due to limitations with OneDrive Personal, the free version of Microsoft's cloud storage, and its lack of support for downloadable files through a share link, OneDrive Personal is not currently supported for hosting .xlsx files.
Google Drive
Click the Share button in the top right-hand corner.
Set your spreadsheet file to be visible to Anyone with the link.
Click Copy link to copy your sharing link.
Switch to Geckoboard, then paste your share link into the Link to file field.
OneDrive for Business
Select your spreadsheet file, then select Copy link.
Set your spreadsheet file access to Anyone with the link can edit.
Select Copy link.
Switch to Geckoboard, then paste your share link into the Link to file field.
SharePoint
Select your spreadsheet file, then select Share at the top of the page.
Set your spreadsheet file access to Anyone with the link can edit.
Under Copy link, select Copy.
Switch to Geckoboard, then paste your share link into the Link to file field.