GS-Calc and ODF file format

GS-Calc can use both its private binary file format and the ODF format as its native file formats.

Typically GS-Calc *.gsc files should be several times smaller than the corresponding ODF files. Thanks to various special compression techniques for certain files that difference can even reach several hundreds times or more. Similarly loading/saving times can be even tens of times faster when this file format is used.
GS-Calc *.gsc files can have any size and can exceed 4GB. There are any practical limits for this file formats except that a single worksheet can use up to about 500GB RAM memory when loaded.

GS-Calc files are plain zip files that contains several zip streams including the workbook data, used images, the list of files (along with optional encryption information), the current file version number, the minimal GS-Calc version required to open it and the thumbnail image. The thumbnail path is: Thumbnail/thumbnails.png.
If the file is encrypted, both the workbook data and all used images are encrypted.

To associate the *.gsc file type with GS-Calc in Windows, you must register that file type in the Windows registry database. To do this, run GS-Calc as an administrator (e.g. right-click GS-Calc shortcut/icon and on the context menu choose "Run as administrator") and use the Settings > Register GS-Calc *.gsc File Type command.
Similarly, if you no longer need that registration, you can use the Settings > Unregister GS-Calc *.gsc File Type command to remove all registry entries added earlier.

GS-Calc uses the Open Document ODF 1.2/1.3 specification. This is a standard file format used by several notable applications.

There are several GS-Calc-specific features that won't be saved when using this file format.
When you save files in this format GS-Calc checks which such features will be skipped and displays this information.

Additionally before saving an *.ods file you can choose whether it's to be used in Google Sheets or Excel.
This option can be necessary as a couple of tens of certain Excel formulas are saved/loaded differently in Google Sheets and Excel.

Some differences that you're likely to come across include the following:

GS-Calc requires array string to be constant like:

=SPARKLINE(D17,{"charttype","column";"ymin", 0; "ymax",2000;"firstcolor","#334960"})

while in Google Sheets you can embedd references

=SPARKLINE(D17,{"charttype","column";"ymin", 0; "ymax",MAX(D17:E17);"firstcolor","#334960"})

To address this you can either enter a constant value, specify all parameters entirely as a reference to a sheet range: =SPARKLINE(D17,F1:G4) or use the VSTACK()/HSTACK() functions to create array parameters dynamically.
Formulas returning arrays/matrices saved by Google Sheets omit the table:number-matrix-columns-spanned and table:number-matrix-rows-spanned attributes that are used to denote spilling formulas.

After opening such files in GS-Calc, for example the last sheet in the "Annual Budget" Google template, you need to update (e.g. F9) it first then delete all cells from "spilled" area below/after the formulas returning the #FILL errors (to let GS-Calc re-create all these cells automatically).