The "api" folder in the "gravityformscampaignmonitor" plugin - even in the latest download from this site (June 22, 2011) - is using "CS_REST_WRAPPER_VERSION" 1.0.0 (see api/class/base_classes.php). However, this is out-of-date... the latest version is 1.0.9.
We noticed as we needed to use the CreateSend API library elsewhere on the site we're developing, but the class-definitions were conflicting with this plugin's. However, if we just used the class from the plugin, certain functions we needed weren't available (eg. CS_REST_Subscribers::update).
To resolve, I copied the latest version of the API over the one that came with the plugin; however, we'd have to do this again if the plugin was updated. So: could you please update the distribution to use the latest API?