Google Grants Eligibility & Program Details
Well, a lot will really depend on HOW you do it.
It is not uncommon for organizations to promote that they will be at a certain event. Just keep in mind that the grant is for your organization and is for you to run mission based ads. So, as long as the ad leads to a page on your website and it is promoting your organizations involvement at the event and not the event itself, you will be fine.
Two similar ads but one is promoting a commercial event and the other is promoting the benefit that the org gets from having a booth at the commercial event.
So when writing your ads and also the content for the landing page, just keep in mind that the focus must be on your orgs participation and the benefit that your org gets from that participation, not the event itself.