The sentence "Compro la gasolina en la estación de servicio" can be modified by substituting "la gasolina" with the direct object pronoun "la". The revised sentence would be:
"Compro la en la estación de servicio."
In this context, "la" is the direct object pronoun that replaces "la gasolina" (which is feminine singular).
The reason we use "la" here is that direct object pronouns in Spanish agree in gender and number with the nouns they replace. Since "gasolina" is feminine and singular, the corresponding direct object pronoun is "la". This construction simplifies the sentence while maintaining the same meaning.