I had to make a minor edit to Gridly theme files to make infinite-scroll plugin work.
I opened up the file wp-content/themes/gridly/index.php
Down near the bottom of the file (line 29 in my version) there’s a line:
<?php next_posts_link('<p class="view-older">View Older Entries</p>') ?>
I wrapped that line in a DIV, so it looks like this:
<div id="older-nav">
<?php next_posts_link('<p class="view-older">View Older Entries</p>') ?>
</div>
Then I set the following through Infinite Scroll Options in my WP admin panel:
Content Selector : #post-area
Navigation Selector : #older-nav
Next Selector : #older-nav a:first
Item Selector : .post
Last step, I followed @svenol’s tip of setting behavior in the options to Masonary/Isotope.