I've been doing web dev for 3 years. Now, I want to do some mobile stuff. After I went through some internet resources, I decided to give Phonegap a chance.
Is it worthy my time?!

Phone gap is good to start. You will be able to use your web dev skills. Just be aware that the more powerful the app the more you will need to get into individual development.

Android Studio is where you should go next. Then look into ios development. It's more nuanced but more lucrative.

I'm going to build a Q&A app. I don't know if it's resources consuming or not because I'm new to the mobile ecosystem.

try ionic framework (it use Cordova/ phonegap). and if your app is not a super 3d game, ionic is sufficient for most jobs.

I second the ionic recommendation. I've used Cordova/Phone gap standalone and it's a pain in the ass

I think you should go with phonegap as you are starting with mobile dev. One thing to keep in mind is when your developing the backend for your mobile app, make the backend as an API so that later on when you decide to do native mobile dev, your backend is already setup and ready to go.

Say what I've exactly done. I've implemented a Rest API already using node/express and mongo

You may have a look at the third way of buiding apps. Native

https://www.nativescript.org