Enter a desired expression, including the leading "=".
For example, to sum A1, A2 and A3 and place the result in A4, enter in the A4 cell:

=A1 + A2 + A3
or
=sum(A1:A3)

To count numbers greater than 2 in the range A1:A100 and place the result in B4, enter in the B4 cell:

=countIf(A1:A100, ">2")

Other examples:

="abc"
={1, 2, 3; "abc", " 'def' ", ' "ghi" '}
=1 + 2
="a" & 'b' & 3
=fv(0.75%, 36, -500, -5500, 0)
=LProg({2,2;1,2;4,0}, {1;1;1}, {14;8;16}, {2; 4},0,,)
=sum(a1:b2, c3, 5:5, 10, sum(20, 30, 40))
=sum(A1:B2, C3, E:E, 10, sum(20, 30, 40))
=index(timeSeries($A$1:$A$100, 3, 10, 0.9), , 1)

Numbers used as arguments cannot be formatted. For example, the expression =$1,000.00 + 1 is invalid. Text strings should be delimited by either single or double quotation marks.

Formulas can contain circular references. For example, if cell A1 contains '=B2 + 1', B2 contains '=C3 + 1' and C3 contains '=A1', then each time you update the workbook, GS-Calc will perform recursive/circular recalculation for those three cells. The number of such circular updates depends on the Recursion level parameter specified in the Options dialog box. Possible values are from 1 to 255.
Circular cell markers are displayed if that level is 2 or higher. To list all formulas with circular references, use the Find All > Circular Formulas command on the Find toolbar.

To browse all available formula categories and examples, use the List Of Formulas dialog box, which is displayed when editing formulas or after clicking the Formula edit field on the toolbar.

Note: Square brackets [,] in parameter lists denote parameters that are optional and can be omitted. For example:
LProg(A, s, b, c, vector, [epsilon], [m])

If your system settings use commas as decimal separators, use semicolons (;) to separate function arguments, or, if you prefer locale-independent (US) settings, select the generic locale via the Settings > Locales > Generic command.

Available operators:

Operator	Operation	Comments	Precedence
=	Equal	Compares numbers or text strings (the comparison is not case-sensitive). Example: A1=4 , B2="abc"	9
<	Less than	Compares numbers or text strings (the comparison is not case-sensitive). Example: A1<4 , B2<"abc"	9
>	Greater than	Compares numbers or text strings (the comparison is not case-sensitive). Example: A1>4 , B2>"abc"	9
<=	Less than or equal	Compares numbers or text strings (the comparison is not case-sensitive). Example: A1<=4 , B2<="abc"	9
>=	Greater than or equal	Compares numbers or text strings (the comparison is not case-sensitive). Example: A1>=4 , B2>="abc"	9
<>	Not equal	Compares numbers or text strings (the comparison is not case-sensitive). Example: A1<>4 , B2<>"abc"	9
+	Addition	Adds numbers.	8
-	Subtraction	Subtracts numbers.	8
&	String concatenation	Merges text strings. Example: A1 & "abc" , "a" & "b"	8
*	Multiplication	Multiplies numbers.	7
/	Division	Divides numbers.	7
^	To the power of	Calculates the power of.	6
-	Negative	Changes the sign of a number. Example: -A1	5
%	Percent	Specifies a number entered as a percentage. Example: 12%	4
_	Intersection	Specifies a (reference to an) intersection of two ranges. Example: 5:5_b:b , a1:a5_a5:a8	3
!	Worksheet reference	Specifies a reference to another worksheet/folder. Example: sheet2!(5:5_b:b)	2
:	Range of cells	Creates a range. Example: r1c1:r5c5 , A1:B5	1

By default, changing any cell's contents updates all formulas and charts in all worksheets. Use the Tools > Automatic Updating command to turn automatic updating on or off.

Use the File > Statistics command.

Select that range. The sum, minimum, maximum and mean values are displayed in the second status bar pane.

or

1. Select a cell range as shown below:
   
2. Click the Formula button on the toolbar and choose the desired function.