How can I get a GMap Marker field to be a target in my Feeds importer?
Mapping targets for fields are not automatically available. For each field, a Feeds target must be written. Feeds targets for fields provided by Drupal core should be provided by Feeds itself, but Feeds targets for fields provided by a contrib module, should be provided by that contrib module.

You can do the following:


Open an issue in the GMap issue queue and request a mapper for the GMap Marker field.
Implement the Feeds target yourself. For documentation, see https://www.drupal.org/docs/7/modules/feeds/the-developers-guide-to-feeds/the-developers-guide-to-feeds#mappingapi. While the process explained there still works, the documentation is slightly outdated. The recommended way of adding a target now is to use the hook hook_feeds_processor_targets() instead of hook_feeds_processor_targets_alter(). Also in the target callback, the language of the target can be found in $mapping['language']. See feeds.api.php for more information.