As Z3R0 said, there is no actual scripting command to get the level of a guild, but this is a simple NPC to get it without having to make an actual command.
prontera,150,150,3 script Guild Level 100,{
set .@guild_id,strcharinfo(2);
if ( !.@guild_id ) {
mes "You aren't in a guild!";
close;
}
query_sql "SELECT `guild_lv` FROM `guild` WHERE `guild_id` = '"+ .@guild_id +"'",.@guild_lv;
mes "Your guild level is "+ .@guild_lv +"!";
close;
}