Thank you all for the valuable feedback.
I mainly just use BIAB for Drums and Bass with occasional keys. I just want simple backings for my guitar. I use VST plugins (MODO Bass and SSD 5.5) in Reaper, so I'm not whether a full GM2 synth is needed... I've experimented with loading the VST's onto each track in BIAB but had a few crashes. I know I could just drag the MIDI into Reaper, but I like the spontaneity of using BIAB.
Loz, your Modo should work on your bass line, just be sure that you are using the VST 2 version.
SSD-5 will not work for your drums because it is not GM compliant. You will need a VST 2 GM compliant drum kit. I think Addictive Drums 2 is VST2 and it has a GM drum setting. Google/Bing and I'm sure you will find others. DO NOT get a VST 3 version as VST 3 plug-ins will not work in BiaB right now. It has been rumored VST 3 is coming soon though.
I hope this helps.