riddle - See me once, see me twice #2

I had a lot of fun with my "see me once, see me twice" question, so I thought I'd make a series out of them.

So here is the next riddle:

see me once, the hammer falls.
see me twice, the gamer calls.

As last time, it starts with only one 2-liner. Additional hints will be given almost hourly.

First Hint:

see me once, and I'm hidden inside
see me twice, game over is implied

Second Hint:

see me once, when I'm lighter than a plum
see me thrice, out of the tunnel you come

Third Hint: (and last one for today)

the "see me once" part refers to a different meaning of me in every two-liner

Last Hints:

the see-me-once in the first hint feels really good when found.
What don't you have in tunnels? It doesn't matter whether you're in a train or a car

See the last one in the series: See me once, see me twice #1

Answer

Are you

G

See me once

the hammer falls -> G, a unit for gravity
I'm hidden inside -> ummm, well...the g-spot?
I'm lighter than a plum -> g, a unit for weight (gram)

See me twice

the gamer calls, game over is implied -> gg ("good game") is often said when a match ends in gaming

See me thrice

out of the tunnel you come -> I think this refers to 3G phone service, which you would lose while traveling through a tunnel