Better late than never.
I’ve had this issue twice. I’m currently using multisite with gravity forms and the paypal addon. It seems that gravity forms had issues establishing its entries in the wp_options table for my site. Some options were there, but the rg_form_currency entry was not there, causing the javascript to pull a NULL value for the decimals entry.
I fixed this by disabling both plugins and re-enabling them, then visiting the “settings” link under the gravity forms plugin in the plugin list. Sure enough, all the options were there, I simply manually saved the options and everything worked fine.
The time I had this issue before, I had to manually assign the default currency type to USD(us dollars) from AUD(australian dollars). I did this either in the options table for my site, or in the common.php file for gravity forms (I believe it was common.php or gravityforms.js, it has been a month or two).
