Rockstar Games answers some questions about Grand Theft Auto V. Several topics of interest are covered, including a clarification of the sort of modifications that will get players in trouble, which focuses on GTA Online:

We have always appreciated the creative efforts of the PC modding community and we still fondly remember the awesome zombie invasion mod and original GTA map mod for GTAIV PC among many other classics. To be clear, the modding policy in our license has not changed and is the same as for GTAIV. Recent updates to GTAV PC had an unintended effect of making unplayable certain single player modifications. This was not intentional, no one has been banned for using single player modifications, and you should not worry about being banned or being relegated to the cheater pool just for using single player PC mods. Our primary focus is on protecting GTA Online against modifications that could give players an unfair advantage, disrupt gameplay, or cause griefing. It also bears mentioning that because game mods are by definition unauthorized, they may be broken by technical updates, cause instability, or affect your game in other unforeseen ways.