If you work with Gravity Forms and build your WordPress themes on Bootstrap, you may find the following function quite useful.
Gravity Forms Filter
Read more about the gform_field_content filter.
add_filter("gform_field_content", "bootstrap_styles_for_gravityforms_fields", 10, 5);
Custom Functions to Add Bootstrap Classes
The following functions use str_replace() which was chosen for this task because it effectively modifies only the small bit of the string we need it to. Gravity Forms [aff] has a reliable structure, so we just insert class names as needed for our common form field types.