1. Equip the gravity gun.
2. Spawn a prop that the gravity gun can pick up (like a chair), so that the claws open up.

3. Open up console and type "ent_remove_all weapon_physcannon" (requires sv_cheats 1)

4. Switch to your physgun, and...
it should be done.
the effect stays when switching weapons.

This method doesn't require cheats and should work on other servers (only you can see it).

1. Equip the gravity gun.
2. Look at something grabable so the prongs open up.
3. Spawn an weapon from the spawn menu that you didn't equip yet.
4. Switch to the physgun (don't switch to gravity gun, or it will reset).
5. The prongs should now be open, you could then use the ent_remove command and delete the gravity gun if you play with sv_cheats 1, so you don't accidentaly switch to the gravity gun.