How to change player's gravity?

Hey,I want to write a plugin to make player to jump higher,how to do?
I have try:
Write a function to change the $gravity in living.php ,but haven't effect.($gravity has really changed)

Isn't this like the jump higher effect?

PHP:

$player->addEffect(Effect::getEffect(8)->setDuration(40)->setAmplifier(1)); 

PHP:

$jump = 1;
$player->setMotion(new Vector3(0,$jump,0));

Thanks for your reply.
How to set this effect Infinite?

Thanks for your reply.
Are you serious? ...

setDuration($time * 20)

just put a really long number,like 100 years

when the displayer can't fit the number, it will display xx:xx