Completed this game last night and really loved the new gravity gun. I won't say much about it as I don't want to spoil the game for anyone but if you've finsihed and want to make this appear earlier in the game here's how:
Enable the console:
Options > Keyboard > Advanced > Enable Developer console
Press the tilde key (usually under escape key) and enter the following:
*wait for it to load*
*wait for load*
You might want to change the d1_trainstation_01 map to something you want. Use the "maps" command for a list.
Thanks to "Strider" over at alt.games.halflife...