Add following line to /etc/udev/rules.d/99-local.rules or create separate rules file for PowerMate:

BUS=="usb", KERNEL=="event*", SYSFS{product}=="Griffin PowerMate", SYMLINK+="powermate", MODE="660", GROUP="video"

You can use here any other group where you are the member.

To use powermate with Gimp, add following lines to ~/.gimp-2.2/controllerrc or ~/.gimp-2.4/controllerrc depending on your Gimp version

(GimpControllerInfo "PowerMate"
    (enabled yes)
    (debug-events yes)
    (controller "ControllerLinuxInput"
        (device "/dev/powermate"))
    (mapping
        (map "button-0" "view-zoom-fit-to")
        (map "dial-turn-left" "view-zoom-out")  
        (map "dial-turn-right" "view-zoom-in")))

You can later easily change your knob controls if you go File → Preferences and open PowerMate tab from Input Controllers section.

Note: Since Gimp version 2.4 zoom doesn't zoom from center of picture but the place where cursor is. This makes it very handy to use PowerMate and mouse together

# modinfo powermate
filename:       /lib/modules/2.6.18.8-0.3-default/kernel/drivers/usb/input/powermate.ko
author:         William R Sowerbutts
description:    Griffin Technology, Inc PowerMate driver
license:        GPL
vermagic:       2.6.18.8-0.3-default SMP mod_unload 586 REGPARM gcc-4.1
supported:      yes
depends:        usbcore
alias:          usb:v077Dp0410d*dc*dsc*dp*ic*isc*ip*
alias:          usb:v077Dp04AAd*dc*dsc*dp*ic*isc*ip*
alias:          usb:v05F3p0240d*dc*dsc*dp*ic*isc*ip*
srcversion:     6D368023674F3EEBDF5C1A7

usb 4-1: new low speed USB device using uhci_hcd and address 2
usb 4-1: new device found, idVendor=077d, idProduct=0410
usb 4-1: new device strings: Mfr=1, Product=2, SerialNumber=0
usb 4-1: Product: Griffin PowerMate
usb 4-1: Manufacturer: Griffin Technology, Inc.
usb 4-1: configuration #1 chosen from 1 choice
input: Griffin PowerMate as /class/input/input4

Bus 004 Device 019: ID 077d:0410 Griffin Technology PowerMate
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0         8
  idVendor           0x077d Griffin Technology
  idProduct          0x0410 PowerMate
  bcdDevice            4.00
  iManufacturer           1 Griffin Technology, Inc.
  iProduct                2 Griffin PowerMate
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           41
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          4 Media Controller
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower              100mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass         3 Human Interface Devices
      bInterfaceSubClass      0 No Subclass
      bInterfaceProtocol      0 None
      iInterface              5 Endpoint 1
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.00
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      74
         Report Descriptors: 
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0006  1x 6 bytes
        bInterval              10
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x02  EP 2 OUT
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0001  1x 1 bytes
        bInterval              10
Device Status:     0x0000
  (Bus Powered)

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