All what I will say needs to be checked/confirmed because I am also a 'Newbie' in electronics.
Ok. I don't know what signal generator you have but usually they are low power TTL output signal. Which means that they can't give enough current for switching ON and OFF high current transistors. Why? Because they have a big capacitor at the gate and you need to charge this capacitor very fast. So usually you have to put an intermediate device between your mosfet and your logic signal. This device is called a "gate driver" it can turn ON and OFF very fastly by charging the gate capacitor with high current.
Here for you have many example of gate drivers at the TI website:
https://www.ti.com/lsds/ti/power-management/mosfet-and-igbt-gate-drivers-products.page
I'll say that any of them will fit with your application as you have not complicated requirements, just find one that is cheap, simple package, low number of components and enough fast for your application. You can find others on linear, AD, digikey, farnell,... websites.
For your mosfet lets go to farnell for example in mosfet section:
https://www.newark.com/webapp/wcs/s...tegoryId=800000055005&langId=-1&storeId=10194
You need Ids>=30A and Vds>=200. So apply the filters and you will find only a few mosfets.
Concerning the 15 to 20ms you need to look into dissipation, the mosfet has a resistor called Rdson which dissipates energy through heat. For example if rdson is
30 mOhm, then you will dissipate 30mOhm*30A=1.2Watt during the 15-20ms. Probably a heat sink could help but it depends on the frequency of your 15-20ms signal.
Another thing be carefull with your 200V 30A, its deadly dangerous.