Hi!

"Extravagant" doesn't only mean "spending too much money" - it could also mean "wearing 'showy' clothing". I think a better phrase to show this is "spendthrift". You could use it either as a noun or an adjective.

When a spendthrift boy is forced to work as a part time cashier
When a spendthrift is forced to work as a part time cashier

The first "Gab" is unnecessary if you are continuing the sentence.

he meets a pretty girl Som and learns that only he can communicate with her.
he meets a pretty girl Som. Gab learns that only he can communicate with her.

The last sentence in the short description is out-of-sequence for a question - and "Gab" should have a capital letter.
Why can only Gab see her?

I think you mean 'convenience store'
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The long description is good except for a couple of points - 'convenience store' again and the grammar of the first sentence.

When you use a participle phase as an adverb, it usually "assumes" the subject of the main clause.
The subject of the main clause is "Gab's parents". Therefore, it is Gab's parents who have been spending money like there's no tomorrow.

You could re-phrase the phrase or the clause - one or the other.
After Gab spends their money like there is no tomorrow, his parents decide to teach him a lesson by no longer sending him money.
This is OK, as it is now two separate clauses, but it feels a little unbalanced as the clauses have different subjects.

After spending money like there is no tomorrow, Gab is sent no more money by his parents, to teach him a lesson.

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