The financial mode of the calculator is very hard to use for someone who did not read the source code. 

For example, the Future Value dialog has 3 text entries, with values defaulting to "0", and a label on their left indicating the variable's name...
However, there is no indication as to what is the unit type or the order of magnitude. Is the interest rate written as "4%"? "0.04"? "0,04"? "4 percent"? "4/100 per year"?
Besides, perhaps there could be a little warning icon when the inputted value is incorrect, with gtk entry icons.

This problem affects all the financial calculation dialogues. An investor friend wanted to do a simple loan calculation, and I sat there dumbfounded as I couldn't figure out a way to get a working result out of gcalctool. 

Since inputting the same variables into a human-readable query using http://www.wolframalpha.com/input/?i=4%25+loan+20+years+200000$ worked painlessly,  I can assume that this warrants a bug report on gcalctool ;)

I'm not 100% sure gcalctool can be really used to compute this kind of query. The point I want to make is that the dialogs are confusing because they leave you wondering if gcalctool really understood the values you inputted.