How to get current customer group id in magento2

Magento\Customer\Model\Session $customerSession using this class you will get current customer group id

protected $_customerSession;

public function __construct(
        \Magento\Customer\Model\Session $customerSession,
    ) {
        $this->_customerSession = $customerSession;
    }

public function getGroupId(){
 if($this->_customerSession->isLoggedIn()):
        echo $customerGroup=$this->_customerSession->getCustomer()->getGroupId();
    endif;
}

NOTE: You only get customer id if customer logged in

you can get group Id by following code

protected $_customerSession;

public function __construct(
        ....    
        \Magento\Customer\Model\Session $customerSession,
        ....
    ) {


        $this->_customerSession = $customerSession;

    }

public function getGroupId(){
 if($this->_customerSession->isLoggedIn()):
        echo $customerGroup=$this->_customerSession->getCustomer()->getGroupId();
    endif;

}

By default, Magento will clear the customer session: \Magento\PageCache\Model\Layout\DepersonalizePlugin::afterGenerateXml.

https://magento.stackexchange.com/a/92133/33057

Take a look:

vendor/magento/module-customer/Model/Context.php

/**
 * Customer group cache context
 */
const CONTEXT_GROUP = 'customer_group';
/**
 * Customer authorization cache context
 */
const CONTEXT_AUTH = 'customer_logged_in';

We can check the logged in customer and customer group:

 /**
 * @var \Magento\Framework\App\Http\Context $httpContext
 */
$isLogged = $this->httpContext->getValue(Context::CONTEXT_AUTH);
$customerGroupId = $this->httpContext->getValue(Context::CONTEXT_GROUP);

Put these code lines in your block.

There is another good explanation here:

https://sohel.dev/how-to-get-customer-id-from-block-when-full-page-cache-enable-in-magento-2/