I have Ubuntu 16.04 LTS running with PHP7 and phpmyadmin installed. However, I get a lot of deprecation notices like:
Depre
Until it gets resolved in dependency itself (in your case the php-gettext) and you don't want to change global PHP settings so that your other stuff is not affected you may want to try to just customize PHPMyadmin's index.php by putting
error_reporting( ~E_DEPRECATED & E_ALL );
somewhere at the beginning or by using
php_value error_reporting 24575
in either .htdocs or virtual host configuration directive. I think the latter option is better.