How to theme markers bubble with Gmap module?
There's a actually a way to do this in Views UI. 


Add all your fields in Views as per usual
On the bottom most field (ideally the one that's the bubble), edit it and click on "Rewrite results".  (Note you can drag / reorder fields to get the right one at the bottom.)
Check the box, "Rewrite the output of this field"
Below that once expanded, you will see "Replacement patterns"
Now add in all the tokens into the rewrite text box. e.g. [title] == Content: Title, [some_other-field] == Content: Some other field, etc... 
Click apply.
Now go in and edit the other fields that you added in to the bottom field and check the box "Exclude from display"
Now go to "Format" probably something like "Gmap" and click on "settings". You will see a "grouping setting
select to group on that bottom most field that you added in all the tokens to. 
Save this and now all those fields should be in the bubble.


Obviously without seeing your view, I was guessing at some of this so you will have to try to make sense of it as it in terms of your view.