Gunclub and multi-thread plaids and checks are quite easy to pair actually. Or maybe I'm just lazy. Besides going for the SF darling of grey pants in all situations, I feel that navy trousers (especially cotton) play that role for cool color jackets/shirts (particularly anything in the grey tones). Brown cotton pants are also an easy go-to choice for anything in warm colors. But, coming back to checks, the easiest is to look at the colors of the threads in the check and simply use trousers in any of those colors + derivations of those in lighter and darker tinges. Some of those choices may be more adventurous than other but nearly all will work from an aesthetic perspective. Only thing I would stay away from in that construct would be trousers in the same color as the dominant thread of the check - so here for example, do not wear with the same tinge of beige as the dominant beige in the coat. But a darker or lighter would be fine. Grey and brown, as represented in the check, would be great matches too - in a range of shades.