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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.
Online Spreadsheet: Link
Here is a link to the Battle Century G (and Z, I guess) Discord Server.