because it's a 105 component group, it means you have cup and cone bearings in the rear hub. it's possible to inadvertently minimize, when reassembling the hub and retruing, dishing the hub, (it that has even been done) the distance between the smallest cog and the driveside dropout, leaving enough clearance for the chain, but causing an excessively acute angle between the smallest chainring and the cassette, especially when using the smaller cogs on the cassette.
as alluded to in previous posts, if the BB is not the same as the one you were using when working properly, i would check the width of the crank when mounted to the new one as compared to the previous one. the new one may be wider, which would cause the problem. also check that no additional spacers were added to the rear wheel between the freehub and the cassette.
if the chainrings were removed check that they, especially the large chainring, were reattached in the proper orientation. some chainrings, i have noticed, have an intentional dish, which could theoretically, anyway, cause the problem if mounted reversed (it would probably interfere at all times, but maybe not).
it can also happen if the size of the big chainring gets bigger when retro fitting. or the small chainring gets smaller, i suppose.
if all else fails, a small spacer between the driveside GXP cup and the frame could be the answer. i'm not very familiar with external BB's so i could be full of it on this one. good luck.