This is the 3373rd most frequent Portuguese word.
grátis
Free; without cost.
Here, 'grátis' functions as an adjective describing tickets that do not require payment.
Eles ofereceram ingressos grátis para o show.
They offered free tickets for the concert.
In this sentence, 'grátis' is used to describe an amount or period (one month) associated with no cost.
Você pode aproveitar esta promoção e obter um mês grátis de assinatura.
You can take advantage of this promotion and get one free month of subscription.
In this context, 'grátis' is modifying 'amostras' to indicate that these samples are distributed freely without charge.