[bigwill33]

Read around these forums. There are literally dozens of people that have discussed this at length by now.

If you do a GM you cannot control what moves the CPU teams make, making it completely different than doing a season.

If you do not turn trading completely off you will have no control over what players are traded between CPU teams or when. They often make very questionable trades.

The CPU teams will sign free agents and make crazy roster moves.

Players disappear and just vanish from the game completely in the mode at times.

If you try and create a player there is a duplicate player glitch.

There is a backup goalie bug where the CPU teams have started their backup for no reason and way more often than their starter. If you do not manually choose your roster your backup will play more often than not, especially if you sim your games.

There is no minor league system in which you can send players down and have them play actual games for their minor league affiliates.

You cannot edit players gear or attributes at all.

And these are just some of the things people have reported that is wrong with GM mode and how it differs from a season type mode. Read around, use this forums search function. It was a highly discussed topic when it was finally discovered that season mode was omitted from NHL 15 next gen.