You do not need the premium version to remove the footer, although it is easiest to remove the footer in the premium version. Click this link and scroll to the section labeled "copyright" for a video tutorial that will show you how to do that with GeneratePress Premium.

To do this in the standard version of GeneratePress, there are two possible standard ways:

Remove the Footer Code from the PHP Source FIle

1. Go to Appearance > Editor and then open inc/structure/footer.php, which should look something like this.  
2. Find the part of that file's code that is used to generate the footer and comment out the line that starts with "echo" and then save the file. It should look something like this:

<?php
// BEFORE
if ( ! function_exists( 'generate_add_footer_info' ) ) {
	add_action( 'generate_credits', 'generate_add_footer_info' );
	/**
	 * Add the copyright to the footer
	 *
	 * @since 0.1
	 */
	function generate_add_footer_info() {
		$copyright = sprintf( '<span class="copyright">&copy; %1$s %2$s</span> &bull; %4$s <a href="%3$s" itemprop="url">%5$s</a>',
			date( 'Y' ),
			get_bloginfo( 'name' ),
			esc_url( 'https://generatepress.com' ),
			_x( 'Powered by', 'GeneratePress', 'generatepress' ),
			__( 'GeneratePress', 'generatepress' )
		);
		echo apply_filters( 'generate_copyright', $copyright ); // WPCS: XSS ok.
	}
}
?>

<?php
// AFTER
if ( ! function_exists( 'generate_add_footer_info' ) ) {
	add_action( 'generate_credits', 'generate_add_footer_info' );
	/**
	 * Add the copyright to the footer
	 *
	 * @since 0.1
	 */
	function generate_add_footer_info() {
		$copyright = sprintf( '<span class="copyright">&copy; %1$s %2$s</span> &bull; %4$s <a href="%3$s" itemprop="url">%5$s</a>',
			date( 'Y' ),
			get_bloginfo( 'name' ),
			esc_url( 'https://generatepress.com' ),
			_x( 'Powered by', 'GeneratePress', 'generatepress' ),
			__( 'GeneratePress', 'generatepress' )
		);
		//echo apply_filters( 'generate_copyright', $copyright ); // WPCS: XSS ok.
	}
}
?>

This will completely remove the footer from your site's source code, which is the best solution (besides upgrading to premium).

CSS Hack

The footer's HTML source looks something like this:

<footer class="site-info" itemtype="https://schema.org/WPFooter" itemscope="">
  <div class="inside-site-info grid-container grid-parent">
    <div class="copyright-bar">
      <span class="copyright">© 2019 Your Website</span> • Powered by <a href="https://generatepress.com" itemprop="url">GeneratePress</a>
    </div>
  </div>
</footer>

You can hide this with CSS by adding this line of CSS:

footer#site-info div.copyright-bar { display:none; }

This solution will hide the copyright bar, although it will still technically be in the page source, just hidden from humans (which is generally good enough).

Related thread for a different WP theme:

• https://www.256kilobytes.com/content/show/6274/how-to-remove-proudly-powered-by-wordpress-and-buddypress

The CIA wants all code in the cloud under their lock and key. They want to ban compilers and make people think HTML is computer programming. - Terry A. Davis