It seems that both .srpm and .gtar files are used for installation packages meant for Linux platform, or at least certain distributions. It may be possible to convert one to another using some kind of transforming tool, but it should generally not be used on system specific packages.
Convert Linux Source RPM Package to GNU tar compressed archive.
We found one software available in the database for this conversion.
Linux/Unix software - convert srpm to gtar on Linux
A free open source operating system based on UNIX
Convert from srpm file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.