Hi Timal, I recommend you to buy the switches at 
https://www.caseking.de. Large selection and a lot in stock. 
You can get CH keycaps for GMMK at DIGITEC, they are in stock (today): 
Glorious PC Gaming Race ABS Keycaps (CH, ISO, 105 keys) They are not the best&nice quality, but there is nothing else that has the correct layout for GMMK.
For the switches I recommend to buy the test-box first and identify the right switch for you: 
Glorious PC Gaming Race Mx Switch Pack I, for one, thought the silver ones were the best at first. But they are too sensitive for typing, as soon as you touch the key, the silver switch is already triggered. In the end, I bought the Kailh Box Brown Switches from Caseking.
Good luck and if you have any further questions, please contact me.