I main GS. On high rank you should use only Lagi S with GS. Prefferably some mix. On my second character I used some nice combo which gave me Speed sharpening, Focus, Weakness Exploit and Earplugs (you'll need some Hearing tali though). Use only Dios GS, nothing comes even close to it on high rank. Don't combo your moves, just charge to Lv3 and unleash big damage. Try to aim in monster part weak to cutting dmg.
Than you should be fine going through G rank. After certain amount of quests you should consider upgrading to Grand Yamato (G rank Jhen) and Diablos X mix. The skills which you're looking for are Rock Steady, Focus, Critical Draw, Sharpness+1 (no weapon slots needed). This is standard GS set. Most people use it with fully upgraded Brachydios GS but you can go for raw and choose Jho GS. There's also Cera (Diablos GS), but I only recomend it to use for veterans. It's purple last only for few hits and have negative affinity. Risky play will pay off if you're skilled enough but it's harder to use than two mention earlier.
Than there're 3 GS that doesn't require Sharpness+1 to get to purple. Alatreon GS, Miralis and Lucent. For alatreon you should make set with Rock Steady, Focus, Crit Draw and Weakness Exploit (with some rare tali you can also get AuS). Other two are fun to toy with but gets easily overshadowed by raw or slime GS's.
You can also make Punishing Draw set and aim for head to get KO, but in 3U monsters are just too fragile to sustain enough to actually pull off even one KO. It's great for soloplay though.