The space-missions MOST, CoRoT, and Kepler deliver a huge amount of high-quality photometric data suitable to study numerous pulsating stars. Our ultimate goal is a detection and analysis of an extended sample of Gamma Dor-type pulsating stars with the aim to search for observational evidence of non-uniform period spacings and rotational splittings of gravity modes in main-sequence stars typically twice as massive as the Sun. We applied an automated supervised photometric classification method to select a sample of 69 Gamma Doradus candidate stars. We used an advanced method to extract the Kepler light curves from the pixel data information using custom masks. For 36 of the stars, we obtained high-resolution spectroscopy with the HERMES spectrograph installed at the Mercator telescope. We find that all stars for which spectroscopic estimates of Teff and logg are available fall into the region of the HR diagram where the Gamma Dor and Delta Sct instability strips overlap. The stars cluster in a 700 K window in effective temperature, logg measurements suggest luminosity class IV-V. From the Kepler photometry, we identify 45 Gamma Dor-type pulsators, 14 Gamma Dor/Delta Sct hybrids, and 10 stars which are classified as possibly Gamma Dor/Delta Sct hybrid pulsators. The results of photometric and spectroscopic classifications according to the type of variability are in perfect agreement. We find a clear correlation between the spectroscopically derived vsini and the frequencies of independent pulsation modes and show that it has nothing to do with rotational modulation of the stars but is related to their stellar pulsations. Our sample and frequency determinations offer a good starting point for seismic modelling of slow to moderately rotating Gamma Dor stars.
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