Which compound type (Samas) is used in the word 'Gaganchumbi'?

The expansion (Vigraha) of the compound word 'Gaganchumbi' (गगनचुंबी) is "Gagan ko choomne wala" (गगन को चूमने वाला - One that touches the sky). Here, the accusative case marker (Karm Karak vibhakti) 'ko' (को) is omitted in the compound word form, making it an example of Karm Tatpurush Samas.