Make a Gravity Form with one label, one field, and one button into a Bootstrap input group with addons. Should be used in conjunction with https://gist.github.com/CFXd/53076583ef1fbaaf959e. Second version removes &:extend() to enable use of media queries.

gf_bootstrap_input_group_with_addons-with_media_queries.less

form {
	@media (min-width: @screen-md-min) {
		.input-group;
		.gform_body > ul {
			.list-unstyled;
			margin: 0;
			> li:first-child:last-child {
				.input-group;
				width: 100%;
				> label {
					.input-group-addon;
					border-right: 0;
					.border-right-radius(0);
				}
				input {
					border-radius: 0;
				}
			}
		}
		.gform_footer {
			.input-group-addon;
			padding: 0;
			border: 0;
			[type=submit] {
				border-left: 0;
				.border-left-radius(0);
			}
		}
		.validation_error {
			display: table-caption;
		}
		.gfield_description.validation_message {
			.hidden;
		}
	}
}

gf_bootstrap_input_group_with_addons.less

form {
	&:extend(.input-group all);
	.gform_body > ul {
		&:extend(.list-unstyled);
		margin: 0;
		> li:first-child:last-child {
			&:extend(.input-group all);
			width: 100%;
			> label {
				&:extend(.input-group-addon all);
			}
			input {
				border-radius: 0;
			}
		}
	}
	.gform_footer {
		&:extend(.input-group-addon all);
		padding: 0;
		border: 0;
		[type=submit] {
			border-left: 0;
			.border-left-radius(0);
		}
	}
	.validation_error {
		display: table-caption;
	}
	.gfield_description.validation_message, .gform_ajax_spinner {
		&:extend(.hidden);
	}
}