Unfortunately, it is. You can't say "lendo é minha paixão". That is usually so when the verbs work as the subect of the sentence.
But it is another thing we can't create a rule. Sometimes you can take that as infinitive or noun.
Ex: I love swimming: eu amo natação/nadar.
So, in Duo's example you can also say "ler é minha paixão", but not lendo. Most of time you only translate as gerund in progressive/continuous actions ;)
A bit.... hope you get through this ;)
Gerund is usually translated as a noun when it comes "alone" as the subject of the sentence (as in "Reading is my passion"). However, if the subject includes more information than just the verb, infinitive is used in Portuguese instead:
- Traveling by car isn't as safe as traveling by plane = Viajar de carro não é tão seguro quanto viajar de avião.
In other cases, we just don't use a noun because it is not common:
- Smoking is a bad habit = Fumar é um hábito ruim.
In this case, a noun for smoking would be "tabagismo", but this word isn't used that much, so we take the verb in infinitive.