Learn Spanish (Latin America): La mayoría de los usuarios leen el foro, ya que es gratuito. - The majority of users read the forum, since it is free.

With collective phrases like la mayoría de (the majority of) followed by a plural noun like los usuarios, you can actually use either a singular or a plural verb! Using the plural verb leen is called agreement by sense and is very common because we are naturally thinking of the multiple users doing the reading. Using the singular lee to strictly match the grammar of la mayoría is also correct, but the plural often sounds more natural in everyday speech.

What does ya que mean here, and could I just use porque instead?

ya que translates to since or given that. It works very similarly to porque (because) to introduce a reason or cause. You could absolutely say porque es gratuito here, but using ya que is a great way to add variety to your vocabulary and sound slightly more sophisticated.

Why use gratuito instead of libre for the word free?

In Spanish, gratuito (and gratis) specifically means free of charge or costing zero money. The word libre means free in the sense of freedom, liberty, or availability, like a free country or having free time. Since reading the forum doesn't cost money, gratuito is the right choice.

Could we use gratis instead of in this sentence?