How grey market orders (OTCBB) are executed ?

I cant see any quote for grey markets

There is any market maker ?

Brokers just combine buy and sell orders from their clients ?

 

They are likely going to send it somewhere that makes markets in OTC. Knight capital is one of them.

 

Quotes are not allowed for gray market.

 

Do you mean "do not exist" rather than "not allowed"? How are quotes "not allowed"? If a market maker starts to publicly quote prices for a grey market security, doesn't the security cease to be a grey market security and become an OTC Pink security instead?

 

It depends on why it’s gray market.

https://www.otcmarkets.com/glossary
Grey Market, "OTC" or "Other OTC" is a security that is not currently traded on the OTCQX, OTCQB or Pink markets. Broker-dealers are not willing or able to publicly quote OTC securities because of a lack of investor interest, company information availability or regulatory compliance.