The comparison of gullywash to pretty much any other custom map development cycle (that's been used in leagues) is a weird one. Gullywash first saw the light of day in the midst of a mapmaking competition which ran in tandem with a custom map pug channel (mpuk pug network). It was a different era of TF2, you could have three standard European pugs going but still get a game going on the custom map pug channel at pretty much any hour of the day, all with a bounty of reasonable players involved at almost all times. The player base has changed, and the way they approach the game and the hours they put into it has changed.
For a little bit more context, the MPUKTF2 pug network had four channels in this period. An open channel, a slightly more selective channel with a few restrictions (pickup2), an invite channel where only elite players could play, and the custom map channel. These were almost always all active in tandem, it was just a better era of the game. It would be a daily occurrence that groups of players would play 10 pickup2 games in a row, get bored and then go and play all the custom maps on the custom pug channel. You could play the game at a variety of levels at pretty much any time of day, and very often you would have two or three overlapping games We just don't have that luxury any more.