Tengo ganas de tacos
 

Everything bolded I didn't fully understand which was pretty much everything she said but I think I got the idea. The tengo ganas de, I really don't understand.
I looked up tener + ganas + de and got this:
tener ganas de
to feel like

Also, I think I've heard dar + ganas before, I am not 100% sure but some sort of expression with ganas..

I've never seen antojar but it looks like its in the conjugation format like GUSTAR. I don't know why SE is used but I am currently studying the uses of SE which are reflexivo, indirect, accidental, impersonal, recíproco. I am assuming it is impersonal.

una conversación por whatsapp.

Her: Jaja no se me antojan
Me: jeje pero después el gimnasio debería comer proteína. no cené hoy
Her: Jaja estos cuestan :showoff::showoff: (the muscle emoticon)
Her: Yo tengo ganas de tacos

Her: Those I don't crave
Me: Hehe but after the gym I should eat protein. I didn't eat dinner today
Her: Haha those cost/worth :showoff::showoff:
Her: I feel like (eating???) tacos ???

The verb is "antojarse". There's no "antojar".

To fancy/to feel like/having a crave.

"Estos cuestan" can be translated as "those are expensive".

I'd say that you translation for "Tengo ganas de tacos" is correct.

Your idea of "tener ganas" is correct.

Tengo ganas de nadar / Quiero nadar = I fell like swimming / I want to swim

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About "Jaja estos cuestan (the muscle emoticon)"

What she is trying to say is that to obtain muscles takes hard work

"Jaja estos -músculos- cuestan (muscle emoticon or her showing muscles)"
"Jaja -to achieve- these takes hard work! (muscle emoticon or her showing muscles)"

Y ... después del gimnasio ...

Thanks everyone. In this example, why del? Is it because I am/was leaving a place? I know when I am using the verb salir, I generally use de+el/la. If I use irse to signify I am leaving, does this follow the rule of using de as well?

Ya me voy del gimnasio - Now I'm leaving the gym
Salgo del gimnasio - I leave the gym

No, it's because 'después de' means 'after' and 'después' means 'afterwards/later' in the marked sentence.
You're newer sentences are correct.