Can I [show ads] or [sell my software for money]?
Yes. See GPL FAQ - Does the GPL allow me to sell copies of the program for money?
You can also show ads, but remember that the GPL requires you to publish source. So if a customer does not like the fact that you show ads, she is allowed to recompile your app without that feature enabled. Or she is allowed to hire a developer to do that for her. She is also allowed to replace your ads with her ads and then to sell her customised version for money. You sold her the software, but the GPL says that she is allowed to change it or to hire someone to change it for her, and to resell her version if she wants.
Must I make my [entire] app open source in that case?
If you depend on GPL software and include it in a way that it becomes a part of your program (e.g. as a module, library, etc.), then that whole program must be open source as well. There could be some exceptions to this, for example if you distribute multiple independent programs together. To find out if this applies, see the GPL Section 5 where it talks about "aggregates".