Here are your parts
Replacement parts for WHIRLPOOL GB2SHTXTS00 | AppliancePartsPros.com

Here is the tech sheet note it is for a S11 could not find one for a S00, but should be very close to yours.
https://www.servicematters.com/d...Sheet%20-%2012827961.pdf
It shows how to force a defrost.

does this have a defrost timer (cant find) or some kind of frost controls???
Defrost is run by the control board

How do I test without a timer????
See the tech sheet.

Here are a couple good links on fridge repair:
Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com
http://www.applianceaid.com/refrigerator.html

If you do not own a meter, I would suggest you purchase a one. You can get a decent digital multimeter for under $20.00. You do not need fancy though it is nice if the leads are a couple feet long.
If it saves ordering one unnecessary part it has paid for itself and you end up owning a useful tool.
Most places will not let you return electrical parts so if you order it, you own it.
A couple things to watch when measuring ohms and continuity
1. Always remove power from the machine otherwise you could blow your meter.
2. Always disconnect at least one side of any device you are checking. This eliminates the possibility of measuring an alternate/parallel circuit path.
3. When checking for closed contacts and continuity use the lowest scale (Usually 200 ohms). Then try higher scales. This scale is 0 to 200 ohms so if the device you are measuring is 300 ohms this scale would show an open circuit which it is not, you are just measuring outside the scale's dynamic range.

There is a good STICKY at the start of this forum about it's use.

Yes it looks like that is where it is.

Remove the covers from the evaporator coils
Try forcing a defrost
If the heater heats then your problem is probably the defrost control.
Be careful that you do not burn your fingers.
Note the defrost thermostat must be frozen for the heater to run, it disconnects power to the heater just above freezing.
If the heater does not heat up during a forced defrost.
Unplug the unit
Check the heater (30 ohms approx.)
Check the defrost thermostat (0 ohms)

I thawed it several days back...wondering if it is gonna freeze up again??? and stop cooling the refrigerator..
For it to be a defrost problem the coils have to be heavily iced/frosted up.
So much so that the fan cannot pull air through the fins.

could it be possible the computer is bad
Since the defrost is OK when forced, the heater and thermostat are OK so all that is left is the control board not going into a defrost mode when it should

or low on refrigerant????? causing the refr. not to cool when it frost up
Depends on how frosted up it was. Usually if you have low freon you will see it will only frost up on part of the evaporator coils. Let it run for a few hours and take a look. The coils should have a light and fairly even coating of frost.

You may also want to check the defrost thermostat sometimes they also use it's value as part of a defrost. If you ran the diags you have probably already done this.