gmod always closing

The problem with crashing would probably be an addon(s) that is very intensive and your computer can't handle it, or you have some conflicting addons that are causing game breaking errors. It is probably the conflicting addons though. If you said it started recently just unsubscribe or disable the addons you have added most recently until the problem goes away. It could also be a bad update to an addon (less likely because if they were competent enough to have it working previously then it is probably working after an update) and you can sort your subscribed addons by recent updates in the workshop.