Actually I m doing gene expression study and thought that we can save our time to directly use gDNA to check gene expression instead of following long protocols of RNA isolation and cDNA making
No you cannot you genomic DNA to look at expression, gene expression is demonstrated by what RNA is present. What you can do is loo at DNA methylation of genomic DNA to look at likely transcriptional activity of certain genes, but this doesnt exactly determine expression, there are many other factors
I totally agree with Michael. As genomic DNA contains both exons and introns you cannot look at the expression of a particular gene. However you can still use your genomic DNA in qPCR which wouldn't give you the exact expression of the gene you are looking for.
P.S: I have been using qPCR instead of conventional PCR to amplify and detect some genes as qPCR is more sensitive than the conventional one.
Gene expression enclosed different steps that may be modulated, including the transcription, RNA splicing, translation, and post-translational modification of particular synthesized gene product (protein).
That is mean genes are expressed by being transcribed into RNA, and this transcript may then be translated into protein. So you can not asses the expression of a particular gene without RNA, so you will have to turn the RNA into DNA (cDNA) for accurate judgment!!
For understanding the gene expression we need to understand the central dogma of biology
The central dogma of molecular biology describes the two-step process, transcription and translation, by which the information in genes flows into proteins: DNA → RNA → protein. Transcription is the synthesis of an RNA copy of a segment of DNA. RNA is synthesized by the enzyme RNA polymerase.
It is possible to study your population of mRNA with sonicated genomic DNA ever radioactive or fluorescent but it is a global and not a gene specific characterization. This has been done in classic overlapping studies which have measured the global populations of mRNA for example of different plant organs. Non repetitive genomic DNA is preferable in this type of analysis.