If you're searching you can quickly find it by using the query "openai chatgpt" to make sure you're downloading the official app built by us.
The direct link to download & install it is here: https://play.google.com/store/apps/details?id=com.openai.chatgpt
This is the icon for the app:
Which countries?
The app is currently available in the countries listed on the ChatGPT Supported Countries article.