define adder
    print $arg0 + $arg1 + $arg2

adder 1 2 3

The following documentation discusses user-defined commands.

Note:
The arguments are text substitutions, so they may reference variables, use complex expressions, or even perform inferior functions calls.

define commandname
Define a command named commandname. If there is already a command by that name, you are asked to confirm that you want to redefine it.
The definition of the command is made up of other GDB command lines, which are given following the define command. The end of these commands is marked by a line containing end.

if

Takes a single argument, which is an expression to evaluate. It is followed by a series of commands that are executed only if the expression is true (nonzero). There can then optionally be a line else, followed by a series of commands that are only executed if the expression was false. The end of the list is marked by a line containing end.

while

The syntax is similar to if: the command takes a single argument, which is an expression to evaluate, and must be followed by the commands to execute, one per line, terminated by an end. The commands are executed repeatedly as long as the expression evaluates to true.

document commandname

Document the user-defined command, commandname, so that it can be accessed by help. The command, commandname, must already be defined. This command reads lines of documentation just as define reads the lines of the command definition, ending with end. After the document command is finished, help on command, commandname, displays the documentation you have written. You may use the document command again to change the documentation of a command. Redefining the command with define does not change the documentation.

help user-defined

List all user-defined commands, with the first line of the documentation (if any) for each.

show user
show user commandname

Display the GDB commands used to define commandname (but not its documentation). If no commandname is given, display the definitions for all user-defined commands.
When user-defined commands are executed, the commands of the definition are not printed. An error in any command stops execution of the user-defined command. If used interactively, commands that would ask for confirmation proceed without asking when used inside a user-defined command. Many GDB commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.