Gain-of-function mutants are usually dominant or semi-dominant, it means you can study them by introducing the mutant gene into a wild type background. This can be done by generating transgenic plants and looking at phenotypes (morphology, growth rate, biochemical pathways, etc....). In my lab we routinely analyse gain-of-function mutations by transfecting plant protoplasts, and since most of these mutant genes have semi-dominant properties, we like to test them by dose-response analysis. Transfecting protoplasts (i.e. by electroporation plasmids harbouring the gain-of-function gene) allows us to use dilution series to check how the effect of the gained function manifests itself in function of gene-product dosage. This means we have to be able to detect the mutant gene product, and we use a bio-assay to follow the altered function. In our case, we are interested in protein secretion, vacuolar transport and ER retention, so our assays are designed to monitor secretion rates, vacuolar sorting fidelity, or ER retention capacity. It is important to realise that in general geneticists prefer loss-of-function genetics because it provides black and white results (you can complement by re-introducing the wild type gene). But if you want to know more about how the gene product actually works, if you want to take the next step after knowing that the gene is involved, then gain-of-function mutations can be very useful and it is often the researchers themselves that can specifically design such mutants based on a working hypothesis. Gain-of-function genetics will become increasingly important and I encourage you to pursue this line of thinking. Good luck. :) 

To above mentioned answer by Dr Denecke you can try other way around also where u can do is, creating mutations in plants and studying them to see any novel gain of functions in comparison with wild type with respect to trait of your interest.

You may expose your probably gain-of-function mutants to several conditions and compare with wild-type plants. And after the identification of gain-of-function, as for example the selection such as in salt stress tolerance, or another type of stress, you can start to characterise by tail-PCR o sequencing to identify the mutation and proceed with its characterisation.

@ Jurgen,

Thanks for the information. I am curious-- is there any example the Gain-of-function mutants are recessive, instead of dominant? You said 'usually' dominant. In this case, we should transform the 'recessive' gene, and find the homozygous plants (say, diploid) in the next generation for studying the 'Gain-of-function' phenomena. Am I correct?

Thank You very much for your valuable suggestions. This would definitely help me. 

@ Yuan-Yeu Yau

Gain-of-function cannot be recessive, so you should study them in the heterozygous state. I used the term "usual" because there are different ways in which gain-of-function can be dominant.

1) Gain-of-function is not always good for the organism, for instance it can be dominant-negative, which means the mutant interferes with the normal function of the corresponding wild type gene, or it influences another gene in a negative way when the wild type gene would not. A good example of this would be an element in a signal transduction cascade which is always active, it "gains the ability" to be active without an inducer so to speak, but this will cause problems when the activity is not desired.

2) A gain-of-function can also be co-dominant, which means the wild type gene can carry out its normal function, but the mutant offers an additional function that does not interfere and may be neutral of beneficial dependent on the circumstances. For instance a receptor that binds to a larger variety of ligands. It doesn't stop the wild type gene from interacting with its usual ligands, but it can bind to other ligands....

3) An example of a positive gain-of-function mutant would be a rate limiting enzyme in a biochemical pathway that can operate at a lower temperature. Normally the organism would stop growing at this temperature, but with the new enzyme it can carry on growing where the wild type stops. But such effect are hardly ever black and white, so instead of dominant, most gain-of-function mutants are semi-dominant, which means gene product dosage dependent. So if you have a lot of the new gene product, it works better than when it is poorly expressed. Remember that when you make transgenic plants, due to position effects and copy numbers that expression is variable from line to line.     

@ Jurgen,

Thanks for your explanation!!