Hmm, Grand Theft Auto III
is a paid app in the Google Play Market, so there should be no need for you to be trying to side-load a paid app's .apk file, right?
If you are having trouble installing the app, you should contact the app's author via their website:
or by email: firstname.lastname@example.org
If I'm mistaken about the side-loading, please send me a PM to elaborate and we'll continue this discussion here, but I'm closing this thread for the time being.