I'm having problems with addons, my /garrysmod/garrysmod has been renamed to /garrysmod/garrysmod_old and when I join the server it says I have to download the maps and content even though I have it.
Look in steamapps/common instead. It should be there now.
Not for everyone. Quite a few people still have it in their username folder and no Garry's Mod folder in their common at all. My self included, yet Garry's Mod works fine. I didn't have to re-download anything or transfer anything across, since there was nothing to transfer to. The only issue I had was Counter Strike Source stuff not working.
It may just be a coincidence, but I've asked quite a few people about this already and the people who seem to NOT have it in the Common folder are those from the UK. Everyone who said they have it in the common folder now are not in the UK. May just be a coincidence, though. Sounds daft, really.