As an aside, Edmonton GDP went down from 2015 to 2016 yet Edmonton salaries led the country in gains from 2015 to 2016. That is exactly what we need to see in Alberta on a path towards diversification. (You can see the details in cansim table
http://www5.statcan.gc.ca/cansim/a26?lang=eng&id=2060011).
One could argue that much of that increase is do to the NDP investing in public services during the downtown to help prop up the economy, but in any case it is a trend that should continue. One thing about GDP in Alberta is that it is obviously tied heavily to the oil industry, which in a sense inflates the real productivity of Alberta workers along natural resources lines. As Alberta gets more diversified, one would expect the GDP to regress towards the mean per capita productivity among Canadian provinces, adjusted for Alberta's unusually young workforce.
Point being, GDP declines in Albertan cities are not necessarily a bad thing, and as long as job growth and salaries keep up, it could be interpreted as successful diversification for Alberta.