Help Needed To Move From AVR To ARM
There's a lot of them. But, off the top of my head, here's some of the larger players in ARM microcontrollers:


Atmel (same manufacturer as AVR; present on the Arduino Due)
Cypress (in particular, the PSoC 4 and 5)
Energy Micro
Freescale
NXP (LPC family)
ST (STM32)
TI


If you're unfamiliar with all of these, I'd recommend you start out with the STM32 family. They're pretty reasonable to work with, and they have a lot of reasonably cheap development boards available, including the Nucleo series.
Not at all. There's an ARM GCC as well, and in fact many of these manufacturers use it as part of their standard toolchain.