Correct

赶快 vs 赶忙

/ gǎnkuài vs gǎnmáng /
Strokes

Definition

赶快 and 赶忙 both an Adverb, which means [quickly; hurriedly].

Example

Only 赶快 can be used in the imperative to describe something that did not happen.
1
Fēi
飞机
kuài
fēi
起飞
le
  
,
gǎnkuài
赶快
dēng
登机
  
!

Board the plane at once, for it is taking off.

2
gǎnkuài
赶快
xìn
sònggěi
送给
  
!

Please sent the letter to him as soon as possible.

Only 赶快 can be used to describe an objective narration of something that has not yet happened.
3
wàng
希望
háizi
孩子
gǎnkuài
赶快
毕业
  
,
hǎo
bāngzhù
帮助
zuòshì
做事
  

I hope my child will graduate as soon as possible and help me with my business.

赶快 or 赶忙 both can be used to describe an objective narration of something that has happened.
4
Jiàn
lǎorén
老人
jiānnán
步履艰难
  
,
gǎnkuài
赶快
  
(
gǎnmáng
赶忙
  
)
chān
搀扶
  

He came up to hold the old man at once when he saw him walking with difficulty.