All examples of numbers, formulas, dates and lists in this Help file use the International/English language settings:

Decimal point: .
Thousand separator: ,
List (formula parameter) separator: ,

Thus, you must enter formulas and numbers as, for example:

=SUM(5589.7, 56.55, field1)

If your Windows language settings changes e.g. the decimal point or the above separators, you'll need to adapt those example accordingly. If your language settings looks like:

Decimal point: ,
Thousand separator: [space]
List separator: ;

then you need to change the notation to:

=SUM(5589,7; 56,55; field1)

By default GS-Calc uses the current user's Windows system Language/International Settings. You can also override this, choosing the option "Settings > Locales > Generic (Fixed)" which will cause using the International/English settings regardless of the Windows language version.