You can use the general form:

=if(condition, true, false)

example:

=if(a1=b1, "Equal", "Not Equal")

The 'true' and 'false' can be statements:

=if(condition_1, true_1, if(condition_2, true_2, false_2))

Your case is similar to:

=IF(A1>B1,"GT",IF(A1=B1,"Equal","LT"))

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A little heartburn is the order of calculation, generally left to right, top to bottom. This means that if one cell is calculated and this result is used in another calculation, the first calculation should be left of or above the second calculation.