On this JollaTides page there is a tutorial on how to install Google Play.
However, on enabling developer mode, I am not asked for a password! So if I type devel-su in the terminal, I cannot answer the the phone when it asks for a password..
developer repo currently is only for Jolla employees (in future maybe for others too), and if you touch that menu currently it will break your store access (even if you enter wrong login details and leave it disabled), and you need to "ssu domain sales" to recover domain back to normal store access.
In coming updates we are fixing it so that people dont accidentally break their store access.