Q:

Analytics for Firebase + Google tag manager + Google Analytics

I'm working with an app and I'm using Firebase , mostly for Analytics. This app also have a website, so what i want to do is sending my data that i'm collecting with Analytics for Firebase to some Property in Google analytics. Can i do this by Google Tag Manager? 
The Flow will be something like this:
Firebase events -> Analytics for Firebase -> Google tag Manager -> Google Analytics

A:

You should always start with a measurement plan that details the data you want to collect. This will help you report the performance of your business objectives and key performance indicators (KPIs) to your stakeholders. If you’re new to measurement planning, check out Analytics Academy for tips on how to create a solid measurement plan.
Then, you can install a basic implementation of Google Analytics via Tag Manager in four steps:

In Google Tag Manager, select ‘Create Tag’ and then choose the Google Analytics tag template.
Select ‘Universal Analytics’.
Configure your tag by naming it (it will default to the tag template name if you do not choose to change it). We suggest naming it "GA Pageview." Then add your Google Analytics property ID or your gaProperty variable. Finally, choose the Track Type, which in this case, should be the most common type of tracking: ‘Page View.’ A pageview tracks each time a browser loads a site page. Other types of tracking include Events, Cross-domain, Social, and Transaction and refer to the type of action you want to track on your website.
Choose your firing triggers. For a basic Google Analytics implementation, select ‘All Pages’. This means that the Google Analytics tag will fire, or execute, on every page of your site.

Google Tag Manager offers a variable type called “Google Analytics Settings” for use in Universal Analytics tags in web and mobile (Firebase) containers. A Google Analytics Settings Variable acts as a central location to configure sets of Google Analytics settings for use across multiple tags.
When you create a Google Analytics tag using Universal Analytics for web, iOS (Firebase), or Android (Firebase), you will be prompted to select or create a new Google Analytics Settings Variable.
Check this: https://analytics.google.com/analytics/academy/

