Can concentration of a protein be determined from a gel quantitatively (rough estimation)?
Yes, you can use SDS-PAGE as a semiquantitative estimate of protein concentration. You need to create a standard curve with a protein of known concentration to compare against. Quantification is done by densitometry. It's a quick and easy process, but keep in mind some limitations:


Band intensity depends not only on the amount of protein but also on the amino acid composition of the proteins and the dye used
The unknown protein and standard should be run on the same gel to remove variations in gels, electrophoresis, staining, destaining and imaging
Densitometry can be quite qualitative depending on how it's done


I'm not sure if you're looking for a protocol, but it's relatively straight forward. I usually load 2, 4, 6, 8 and 10 uL of 100 ng/uL BSA as a standard and 2, 4 and 6 uL of the protein of interest (based on past experience with my proteins and their purification). The gel is stained with Coomassie and the band intensity estimated with a program called ImageJ. A standard curve is created by plotting intensity as a function of mass, from which the concentration of the unknown can be found.