The question cannot be answered, because the underlying assumption in the question
How do non-Bavarians respond?
that a person's behavior can be sufficiently be characterized as "non-Bavarian" in this question, is just not valid. The response on "Grüß Gott" is more a matter of individual style, preferences and character.
For example: In talks with unknown counterpart, I tend to copy the behaviour of my partner. So, if someone says "Grüß Gott", I respond "Grüß Gott", if someone says "Moin", I respond "Moin", if someone says "Servus", I say "Servus" and so on.
But for sure there will be people who find it important to mark their own origin and their own standard and always respond in the way they are used to in their group, their region, etc.
And there will be people who adapt to the region the conversation actually takes place - so in Bavaria or Baden-Württemberg they might respond with "Grüß Gott", and in - say - Hannover they respond with "Guten Tag".
The following is written in reaction to the comment of rexkogitans:
These three examples are just examples. There may be many other reasons why people respond in the way they respond. It can be a matter of habituality. Or some people might refuse to answer "Grüß Gott" because they are atheists and have a problem in responding "Grüß Gott". There are as many differents reasons as people are out there.