However, there is a catch, currently, technology has not advanced to a point that most commercial ad servers can locate where a user is reliably without the user actually inputting their location manually. Since most users will not voluntarily give their location just to be served an ad, there is a high chance that location targeting done the traditional way that most advertising servers have done in the past, e.g. IP targeting, will be correct. In fact, many mobile data services tunnel their access in ways that do not consistently give the actual location of the user, and could be off as far as even being in the wrong state. This is in contrast to IP targeting desktop users, where IP targeting has been honed to a point that it is fairly reliable -- in other words, that a user whose IP says they are in Japan is in fact sitting in Japan.
The solution is to have the ad server write the location of the page (if relevant) or the location of where the user is registered (if the site requires or uses registrations) into the ad tag itself. This is called many things in different ad servers, but in DFP, this is called key-value targeting. So, if you have a page that is for a restaurant in Las Vegas, write "metro=lasvegas" into the ad tag. If you have a site that requires or cookies users who are registered, and they put their home location as New York, write that into the tag. If the user did give their location when they signed into the app or mobile web site -- write that location. This can then be targeted instead of trying to sniff by IP.
Will this work in all cases? No. But it is a best-effort implementation for an issue that currently limits the entire advertising industry. Future upgrades of various ad server products may fix this, but for now, this is the best that can be done given the unique limitations. This allows publishers to have some kind of reliability around Geotargeting in their mobile inventory.
If you are a publisher, your solution is above. If you are an advertiser, ask if the site or property you are advertising on has the solution above, if Geotargeting is one of the criteria you require or seek. If not, you have a ready-made solution you can suggest.