Hi Cin-Ty, well done! Looks like a Kelp. According to the Dittman/Cardiff paper in Birding, first generation Chandeleur Gulls (KelpxHerring) would have a mantle shade similar to Laughing. You can find a pdf of that article here
2nd generation backcross hybrids (Kelpx1st gen hybrid) can have mantle shade approaching pure Kelp. However, unless I'm not reading it correctly, these hybrids often show large square mirrors on P10 which your bird doesn't show. Looks very good for Kelp.
By the way, this is a good link to bookmark:
This interesting worn gull, which in my eyes looks good for Kelp, is moulting itīs primaries, with innermost p1+2 newly grown and with broad white tips. P3+4 dropped while the rest are old. So if a good Kelp it would have adopted itīs moult cycle similar to to N. Hemisphere gulls, although this individual might be a little late compared to the average LWHG give and take variation of course. Pattern of primaries, small white mirror on p10 and pattern on p6+5 looks good for Kelp.
just a clarification. In the last set of photos
the last 4 photos are of a Lesser Black-backed Gull that the photographer inadvertently put on his webpage. Just so nobody gets confused.
I would say that the last 4 images in Marks (as usual excellent photos) looks more like a GBBG on eg colour, bill sice and structure, perhaps not fully adult.
(As an aside, I'm actually not certain of what would be the "normal" moult cycle of a Peruvian bird. The birds breeding on guano islands have highly variable breeding cycle, and one could expect the moult cycle to adjust on the breeding cycle... Or not? Assuming a bird could cross through Panama, these are by far the closest populations from Texas.)
'Looks like a Kelp to me too.