most of the time,when you have a rom that clockwork will flash and RA_GNM wont,it is because clockwork does not do signiture verifications by default,as RA does. in these cases simply toggling the signiture verification off will let the rom install without having to fool with clockwork.
if RA still fails,hopefully the rom youre installing will have 2.5.x and 3.x downloads. flash 3.0.0.8 and then use the 3.x download. ive only come across one(warmtwopointtwo) that ive had to use clockwork instead of RA. hopefully RA will have an update soon that will let it install roms with the 3.x type scripts.
edit: if you are going to use RA(or clockwork for that matter) without a signiture verification,make sure you check your downloads by comparing the md5 sum of your download against the md5 sum listed by the rom developer. if the sums match you can be reasonably sure that youre not attempting to flash a corrupt or incomplete download.