Make a new number variable, we'll call it randomText. Then on gameover we set the value of it to floor(random(10)) - that will allow us to put in 10 random phrases, just change that number if you want more or less.
if randomText = 0: set text to "great job"
if randomText = 1: set text to "you are awesome"
if randomText = 2: set text to "no doubt you are the best"
Just bear in mind that with floor(random(10)), your ten possibilities are stored in 0-9, value 10 will never come up, because the floor() function is rounding numbers for you