I Write from Italy, is it possible to use rezgo from my country, for tours and activities in Italy?
There is no problem using Rezgo in Italy.

Any problem for bookings with Credit Cards or Paypal?
You will want to be able to accept credit card payments online.  In Italy, we support Stripe: https://www.stripe.com/it as well as Cartisi X-Pay.

Does it exist a plugin of Rezgo for wordpress?
Yes, you can find the latest version of the WordPress plugin here:
/rezgo-wordpress-plugin-version-2-0-beta/