The Beatles use the form "will" and "be going to" in its colloquial version: "gonna".

We usually write "be going to", but when talking, we can say "gonna".

1, 2, 3...
You're gonna lose that girl (yes, yes, you're gonna lose that girl)
You're gonna lose that girl (yes, yes, you're gonna lose that girl)

If you don't take her out tonight
She's gonna change her mind (she's gonna change her mind)
And I will take her out tonight
And I will treat her kind (I'm gonna treat her kind)

You're gonna lose that girl (yes, yes, you're gonna lose that girl)
You're gonna lose that girl (yes, yes, you're gonna lose that girl)

If you don't treat her right, my friend
You're gonna find her gone (you're gonna find her gone)
'Cause I will treat her right, and then
You'll be the lonely one (you're not the only one)

You're gonna lose that girl (yes, yes, you're gonna lose that girl)
You're gonna lose that girl (yes, yes, you're gonna lose that girl)
You're gonna lose (yes, yes, you're gonna lose that girl)

I'll make a point of taking her away from you (watch what you do), yeah
The way you treat her, what else can I do?

(You're gonna lose that girl)
(You're gonna lose that girl)
You're gonna lose that girl (yes, yes, you're gonna lose that girl)
You're gonna lose (yes, yes, you're gonna lose that girl)
You're gonna lose (yes, yes, you're gonna lose that girl)

I'll make a point of taking her away from you (watch what you do), Yeah
The way you treat her, what else can I do?

If you don't take her out tonight
She's gonna change her mind (she's gonna change her mind)
And I will take her out tonight
And I will treat her kind (I'm gonna treat her kind)

You're gonna lose that girl (yes, yes, you're gonna lose that girl)
You're gonna lose that girl (yes, yes, you're gonna lose that girl)
You're gonna lose that girl (yes, yes, you're gonna lose that girl)

GONNA= (coll.) The standard form to write is "going to", but in colloquial speech it is common to pronounce it "gonna", and in colloquial writing (especially in songs and conversation transcriptions) you can also spell it "gonna".

TAKE HER OUT= Take her to places to have fun (cinema, restaurant, countryside...)

CHANGE HER MIND= Think differently, have a different opinion about something (in this context, a different opinion about you, that is, she may stop wanting to be your girlfriend)

TREAT HER KIND= Be nice to her.

FIND HER GONE= If you find her gone, one day she is not there, she's gone, she's not your girlfriend anymore (because she will be my girlfriend).

'CAUSE= (coll.) Because.

LONELY= Feeling bad because you are alone.

MAKE A POINT OF + -ing= Be determined to, make a firm decision to do something.

As usual with The Beatles' songs, the lyrics are simple, short and repetitive. This is a boy who likes his friend's girlfriend and he tells him: be careful because if you don't treat your girlfriend as she deserves (and I think you aren't), I will steal your girlfriend (because I am better than you and I will treat her better than you do).

In short: Ooh, nice girlfriend you have. Watch out!