The Gran Telescopio de Canarias Adaptive Optic System (GTCAO) is designed to provide nearly diffraction-limited images to GTC. GRANCAIN (GRAN CAmara INfrarroja) is a first light imaging instrument in J, H, and K infrared bands, and it will be integrated into the Nasmyth focus of GTCAO, contributing to carry out his acceptance tests. The instrument aims to capture NIR (Near-Infrared) diffraction limit images for a field of view of 22x22 arcsec operating up to seeing conditions of 1.5 arcsec and zenithal distances up to 60deg. The instrument has a telecentric optical design based on a collimator-camera with a 2:1 magnification, with a cold stop and the filters located in the collimated beam. The detector is a 4Mpx Hawaii-2 PACE Teledyne (H2P), which operates at 77K. The instrument is composed of a rectangular aluminum 6061-T6 cryostat cooled by a closed Gifford-McMahon helium cycle cryocooler with an optical bench where the entire optical train is mounted. The opto-mechanical system consists of two filter wheels, containing three wide and three narrowband filters in the J, H, and K bands in charge of selecting the wavelengths of the science images. Thanks to a wide background knowledge built up at the Instituto de Astrofísica de Canarias (IAC) and the use of commercial elements, the instrument development plan has been reduced to one year and a half. The article contains a general description of the design, fabrication, integration, and testing of the opto-mechanical elements, cryostat, cooling, and anti-vibration systems.