how do the += work

[ebon pier]

i was working on a project that accelerating in this state : velocity.y = (velocity.y + gravity) * delta
but accelerated when in this state : velocity.y += gravity * delta
what's the difference and are there any other versons of += (also sorry I'm just starting to leran gdscript)

[midnight loom]

velocity.y = (velocity.y + gravity) * delta is different from velocity.y += gravity * delta.

delta is the time thats passed since the last frame of _process or _physics_process which is used to keep movement consistent even if the game lags or anything, and is very small.
velocity.y = (velocity.y + gravity) * delta is taking the entire sum of the current y velocity plus the gravity amount, and multiplying that entire sum by delta, and assigning that value.
velocity.y += gravity * delta is taking *just *the gravity amount and multiplying it by delta, and adding that value to velocity.y.

in other words, += is just a shorthand for saying "take the current value, add some amount to it, and reassign that new value to the variable.

there's also other versions that are similar for all the other typical math operators:
-= for subtraction
*= for multiplication
/= for division

hope that helps!

[sleek meteor]

basically
velocity.y += gravity * delta
is the same as
velocity.y = velocity.y + (gravity * delta)