$1.45 for 1 GBP. It used to be $2.00. If it went back to $2.00, it would be an increase of 38%. Does my investment really just increase in value to 38%? What sort of hidden fees are there, etc. And how likely is it that GBP would even return to that value.

Answer
The leverage for Forex is 100:1. This means that you only need a $1000 to control an amount $100000.
1 pip is 0.0001 and is worth USD $10 for GBP/USD currency pair. If GBP/USD goes up from 1.45 to 2, it will be a profit of 5500 pips * 10 = USD $55000. This makes the return to be 5500% for the initial margin of USD $1000.
There is no commission for Forex but you need to take care of the bid/ask spread in the trade. When you roll over to the next day, you will either receive interest or pay interest depending on which currency have the higher interest rate. If you buy the currency with the higher interest, you will receive interest when you roll over.
IMHO, it is unlikely will GBP/USD return to $2 level in the short term as the economy outlook is uncertain.

Trading Forex--is 100% gain in one year possible?

Q. This blog claims it is possible, trading only with $8.
http://100percentforexchallenge.blogspot.com/

What do you think?

Answer
Don't think 100% is anything to shout at. Generally people target for a lot more.