Hi @prateekgrwl and welcome to the community 
Actually GA is able to track the Unbounce variant. As long as you use the native GA-integration found under “integrations”.
Let’s say you are running your test on the URL ending with /example/. When using the native integration you’ll be able to see the different variations like this in GA:
/example/a
/example/b
I recommend setting a goal in GA that will track the button click. Unbounce creates events out of the box when using the native script, but I always define the events myself for improved control. In your example you could add a script (before body end tag) on the landing page variants like so:
<script>
eventTracker.trackOne('#lp-pom-button-751', 'button', 'click', 'buy');
</script>
Replace button id with your current button.
Now create the goal in GA using event
Category=button
Action=click
Label=buy
If you have more than one button you could even assign classes to the buttons - then set the script to fire on class instead of button id.
In this case use the following script to firing GA events:
<script>
eventTracker.trackOne('.button-buy', 'button', 'click', 'buy');
</script>
Hope this helps!