Some browsers can display PDF files incorrectly. For all PDF downloads, please download them to your computer and view them with any PDF viewing software (I personally use SumatraPDF, just remember to enable bookmarks!) instead of viewing them from your browser.
You can get digital versions of BCG and BCZ here and here respectively.
You can get physical versions of both here.
System Reference Documents
The Battle Century G SRD has the whole of the game's rules.
The Battle Century Z SRD has the universal rules content in the expansion.
Download PDF from Mediafire or Depositfiles.
Character and Mecha Sheets
To use the Online Spreadsheet go to File -> Make a copy and you will have your own sheet to edit as necessary.
Printable PDF: Download from Mediafire
ZeronosVega's Fillable PDF Sheets: Google Drive.
ZeronosVega's Stat Calculator: Google Drive.
Online Spreadsheet: Link
Here is a link to the Battle Century G (and Z, I guess) Discord Server.