This butterfly-like Witch attacks by throwing red couches at a decent distance. Her attack can wipe out most small cats and some medium cats in one hit, so it is still best to keep an ongoing meatshield of smaller cats to protect any larger heavy hitters.
Strategy
It is recommended to use any cats that outrange her and keep them protected by using any smaller and cheaper cats. In the long run, defeating Gertrud should be not be much of a sweat.
One way to speed up the fight is by using both Li'l Madoka and Li'l Homura to activate the My Very Best FriendCombo, which will greatly amplify the effect of the "Witch Killer" ability against Gertrud.