Gromyko was only as conservative as Ligachev was conservative, but they both wanted to reform the system and strip out all the corruption and imbalance in the economy. So perestroika would have still been around in some form.
Gromyko would have needed a different job other than Foreign Minister for a while in order to build his power base. You would also need to find a way to break the relationship between Andropov and Gorbachev, so that Gorbachev is not the inheritor of his mantle, and break the alliance between Gorbachev and Gromyko. Gromyko supported Gorbachev's ideas and was the person who nominated him to run the funeral of Chernenko over Grishin, who was the real conservative.
As to glasnost, Gromyko was open to specific, limited proposals, but was not in favour of it as an overall policy. He was the originator of detente, as so would also have pursued peace with the United States.