Command 'gradle' not found. How can I run the gradle command from any directory?

I am trying to learn gradle build.

I followed these instructions on gradle.org.

To verify it i am issuing command.

gradle -v

I get this output:

Command 'gradle' not found, but can be installed with:

sudo apt install gradle

But after issuing this command from the installation link

$ export PATH=$PATH:/opt/gradle/gradle-4.10.2/bin

the above command shows gradle version in my computer.

But the problem is once I close the terminal and issue the command I get the error command not found, and once again if I issue the export PATH... command, then the gradle command starts working.

So how do I add this path permanently so I can issue the gradle command from anywhere?