Enter a desirable expression including the leading "=".
For example, to sum up 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 either by single or double quotation marks.

Formulas can contain circular references. For example, if the A1 cell contains '=B2 + 1', B2 contains '=C3 + 1' and C3 contains '=A1', then each time you update the workbook, GS-Calc will be performing 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 >= 2. 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 that is displayed when editing formulas or after clicking the Formula edit fields 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 (setting) uses commas as decimal separators, use semicolons (;) to separate function arguments or - if you prefer to use system independent (US) settings - select the generic locales 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		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	Example: -A1	5
%	Percent	Specifies a number entered as a percentage. Example: 12%	4
_	Intersection	Specifies an (reference to) 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 contents updates all formulas and charts in all worksheets. Use the Tools > Automatic Updating command to turn on/off automatic updating.

Use the File > Statistics command.

Select that range. The sum, minimum, maximum and mean values are displayed in the 2nd status bar pane.

or

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