The original sentence "Compro la gasolina en la estación de servicio" can be transformed using the direct object pronoun for "la gasolina," which is "la" (since "gasolina" is a feminine singular noun).
The modified sentence would be: "La compro en la estación de servicio."
In this sentence, "la" is used as the direct object pronoun that replaces "la gasolina." We use "la" because it corresponds to the feminine singular object being referred to in the sentence. Direct object pronouns in Spanish agree in gender and number with the noun they replace. Since "gasolina" is feminine and singular, we use "la."