What does 00G mean in mobcop?

00G is MOS duty immaterial SPC (Join to see) There should be a duty description after the 00G
Here are some examples
00F3X RSP Instructor
Performs duties as an Instructor for a Recruit Sustainment Program (RSP) detachment; responsible for the health, welfare, and training of a 118 Soldier detachment; acts as a positive role model for all RSP Soldiers by portraying the seven Army values; provides instructions in all common skills and ensures strict adherence of safety procedures during training; conducts all training to standard using the assist and insist method of training; coordinates all recourse requirements to support the program of instruction.
00G4O/Observer Controller/Trainer
Performs duties as an Observer Controller/Trainer for Training Support Battalion; responsible for the health, welfare, and training of deploying units; acts as a positive role model for all deploying Soldiers by portraying the seven Army values; provides instructions in all common skills and ensures strict adherence of safety procedures during training and live fire exercises; conducts all training to standard using the assist and insist method of training.
Asst Processing NCO/00G20
Receives Soldiers for In-Processing brief at reception station; assists with scheduling and processing procedures daily; processes schedules in a timely manner; provides leadership to her squad with mentorship and counseling; responsible for an effective training environment in the field and classroom.
http://www.armywriter.com/NCOER/00G.htm

Immaterial

There is a DA guide to all MOSs but I don't have access to that right now. However, Wikipedia (for what it's worth) says that is it a MOS Immaterial USAR Soldier. That would make sense based on the first 2 digits. So as long as you are the correct grade, you should be able to apply. Tour of Duty will not accept your application if you do not meet the position requirements. That will be ultimate way to know if you can apply. Go for it!