A little help with terminology

rates-how fast you gain base exp/jobexp/item drop compared to kRO (and some iRo servers, I think some have a bit increased rates).
customs-items that are available on server but not on kRO/iRO
max level, stats etc...some, mostly mid and higher rates (take a look around and you will see what does it mean) increase these, normally max stat is 99 and so is base level. iRO sticks to kRO, so there is no problem with that for you.
Limited play time and you don't like to pay for playing? It's your choice, but I think iRO is not for you. There's a whole world of private servers, and most of them are free to play.
Now let's get to GS. It's a fun to play class, but it's really only for fun. It's no match for normal rebirth classes. If you wanna have some fun and don't want to compete with other people, it's ok for you. But most MMORPGs are sooner or later about PvP and such stuff, and GS is not the best for this.
Most used builds are revolver and shotgun ones. In every case, you need to have dex rounded to multiplies of 10 including equip, and as high as possible. Then there are two possibilities, agi and vit build, just get it to max, and those few remaining stat points will go to int. Agi is pure PvM, and I wouldn't recommend even entering PvP or WoE with it. Vit builds are meant for PvP a bit too, and are based around smart using of your skills.
Revolver builds aim for Desperade and Rapid Shower as their main skills, shotguns go for shotgun frenzy.
I won't give you exact stats, use simulator at
ro.doddlercon.com/db/calc/ to make the build yourself depending on equip you can get. For skills, use skill simulator at
http://ratemyserver.net/skill_sim.php.