Palla,
I will try to answer what I can
How can this be useful in mobile applications?
For usage of grails in mobile applications, please see this good article
http://www.ibm.com/developerworks/java/library/j-grails06178/index.html
What is the difference between this and Ruby on Rails?
Grails and Rails have similar
philosophy. However, Groovy has easier learning curve for Java developers. Hence it makes Grails a natural choice for you if you are coming from a Java background.
4. What is the commercial potentiality of this technology?
People are beginning to use Groovy in Java projects for smaller tasks such as
Unit testing. Groovy is not yet the main language on large scale projects where Java is more dominant. However, you will be able to find many examples of Groovy and Grails usage in smaller/medium applications. As far as the job market goes, it is unlikely that you will get your next job/assignment solely due to your knowledge of Groovy/Grails