I've had to give some thought to this question in the past, while purchasing a keyboard for someone else. So please excuse the triple-posts.
If you do a search for "Aliaz 70G" on Youtube, you will get a wealth of videos.
In the Cherry MX world, Silent Reds and Silent Blacks are bush-league 'silencing.' The Leopold keyboards are pretty quiet on their own, and with the Cherry 'silent' switches, are about 5 db lower in volume than the regular red/black linears. The Youtube video I posted above, concerning the Silent Reds, has a more extended test showing the relative volumes of these switches.
You can also get into the "big leagues" of silencing with a custom keyboard. Watch the typing tests at the end and prepare to be amazed:
But that is probably beyond your budget. And, honestly, beyond your silencing needs. Those keyboards are quieter than general office background noise, so are unnecessarily silent.
The switch tester doesn't lie. (
) All regular MX tactiles are fairly noisy. The Aliaz are much quieter. That's why I recommended it. It's really all you need. If you can get some dampening foam in a GMMK, the Aliaz will do the job.
I also recommended silenced 45 gr Topre. It's tops. It really is. But you can only get the reasonably-priced
silent Leopold variants in non-standard formats (FC660C, FC980C), or the super-expensive Realforce R2 PFU edition, which is just overpriced.
So, as others have stated, you can probably get away with regular unsilenced Topre. It's usually cheaper than silenced Topre. I'd say the sound level for unsilenced Topre is about the same as Cherry MX Silent Red / Black. About the same as regular office rubber dome keyboards, so it won't be considered loud by co-workers.