Here is your parts breakdown
Model GB22DKXGW00

You could replace all three. Looks like your total bill would be about $80.00.
Unless of coarse if it is an uncommon problem like a bad connection in which case you have wasted $80.00. Personally I would buy a meter, see below.

Here is a diagnostic which may save some dollars
Mark the timer shaft/cam and the body with a line using a felt tipped pen.
Let the unit run for a couple hours
if your lines are still lined up it tell you that the timer is not advancing and needs replacing.

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.