If block was ever decent this would make sense, but Gwen gets like 2 block sometimes I guess where Ceridwen gets health for every soul stone which is generally part of casting powerful spells anyway. Death coil + hex is crazy, of course, but even without that channel + starting spell can produce 3 soul stones in a turn, and those stones also let you cast powerful spells like the 4 damage 1 weaken starting one, or other better options. Gwen getting three block on a turn usually comes with almost no upside-- probably some thorns, which are lost at end of turn and usually do 2 damage-- and then all that block is lost. That's not to say Gwen is unplayable-- she's great and her starting spells are better defensively than Ceridwen's starting spells because block+weaken means you do effectively have 3 block almost every turn as long as you get moderately lucky, and you can put up with damage on the runs where you don't get lucky. But her main advantage is that not having soul stone spells in the drawing pool makes getting certain other useful spells-- most importantly high-damage spells like Air Blast-- more likely. So Gwen needs to kill all the enemies before they can attack, mostly, managing health as a resource to deal with bad luck while Ceridwen can comfortably coast and do whatever, relying on her mid-late game defensive power to overwhelm anything the enemies can dish out.
If you want block to be relevant, we need better defensive spells, or block on offensive spells. Right now defensive spells that are utility (like channel) are the only ones I ever take. Block doesn't feel like it makes progress, and there's rarely spare mana for things that don't make progress. Maybe a Toughen+block spell where toughen is like the opposite of strengthen (but presumably wears off because its a player thing) would change up how I play Gwen, but it would probably be kinda overpowered. Fundamentally, a defensive spell good enough to be slated into a build is kinda overpowered since it's pretty anti-synergistic with hyper offense and if it's good enough to overcome that, then its good enough you'd want it in every deck. I think if you gave defense more mechanics so that they could synergize together the way different offensive spells can complement each other that would be the best option, but since weaken is classified as an offensive effect the design space is pretty limited-- offensive spells have lots of possible targets to mix things up with but defensive spells can only target and be based off of the player, and the only defensive effects currently in the game are heal and block (other defense cards are really doing some kind of utility feature helping you make progress in an unusual way at the cost of not doing anything else, like channel and that board-wipe spell).
-edit- Turns out I was misremembering the default attack spell for Gwen. No weaken. Man she sucks XD