Style Gravity Forms with Bootstrap 3 (SASS) - From: http://roots.io/style-gravity-forms-with-bootstrap/

gravityforms.scss

.gform_wrapper ul {
  @extend .list-unstyled;
}
.gform_wrapper li {
  @extend .form-group;
}
.gform_wrapper form {
  margin-bottom: 0;
}
.gform_wrapper .gfield_required {
  padding-left: 1px;
  color: $state-danger-text;
}
.ginput_container input,
.ginput_container select,
.ginput_container textarea {
	@extend .form-control;
}
.ginput_container textarea {
  height: auto;
}
.gform_button {
	@extend .btn;
	@extend .btn-primary;
}
.gform_wrapper .gfield_error {
  .gfield_label {
    color: $state-danger-text;
  }
  input,
  select,
  textarea {
    border-color: $alert-danger-border;
    background-color: $alert-danger-bg;
    color: $alert-danger-text;
    @include form-control-focus($alert-danger-text);
  }
}
.validation_error {
	@extend .alert;
	@extend .alert-danger;
}
#gforms_confirmation_message {
	@extend .alert;
	@extend .alert-success;
}