Elution Rate - Alkanes VS Alkenes in Gas Chromatography
Chromatography in general is a separation method in which the partition equilibrium of a compound between two phases is permanently disturbed since one of the phases is changing/exchanged (mobile phase).

Apparently, the mobile phase in gas chromatography is the carrier gas.
The stationary phase isn't just a hollow tube but a capillary covered with various differently functionalized materials, that may be rather unpolar (squalane), more polar (polyethylene glycol) or polar and shape-selective (cyclodextrines). Other very common materials are alkyl-substituted polysiloxanes, such as polydimethylsiloxane.

Using the latter stationary phase, Alan Katritzky and coworkers published QSPR: the correlation and quantitative prediction of chemical and physical properties from structure, Chem. Soc. Rev., 1995, 24, 279-287 (DOI, PDF). Here, the retention times for dodecane and 1-dodecene were given as 4.59 and 4.45 min, respectively.

Ladislav Sojak published a freely available 35-page review Separation and identification of isomeric hydrocarbons by capillary gas chromatography and hyphenated spectrometric techniques in Petroleum &amp; Coal, 2004, 4(3), 1-35 (PDF). You might want to have a look at this article on the effect of having non-terminal $\ce{C=C}$ double bonds and cis-trans isomers of longer alkenes, which therefore have significantly different shapes.