It may be worth completely stopping GP from outputing the standard meta, and adding it yourself:
add_filter( 'generate_header_entry_meta_items', '__return_empty_array' );
add_action( 'generate_after_entry_title', function() {
?>
<div class="entry-meta entry-meta-container">
<div class="entry-meta-gravatar">
<?php echo get_avatar( get_the_author_meta( 'ID' ) ); ?>
</div>
<div class="entry-meta-info">
<div class="entry-meta-author">
<?php generate_do_post_meta_item( 'author' ); ?> in <?php generate_do_post_meta_item( 'categories' ); ?>
</div>
<div class="entry-meta-date">
<?php generate_do_post_meta_item( 'date' ); ?>
</div>
</div>
</div>
<?php
} );
Then your CSS:
.entry-meta-container {
display: flex;
align-items: center;
}
.entry-meta-gravatar {
margin-right: 10px;
}
.entry-meta-gravatar img {
width: 50px;
}
.entry-meta-info {
display: block;
}