My initial post was actually asking in which frequency band you werre working. I assume you’re talking about GSM900 and UMTS900 co-existance ?
In this case, 3GPP said that a guard band of 300 kHz is enough. One GSM chanel = 200kHz, so you’re fine if you keep 2 arfcn (400kHz) separation between the start/end of UMTS 5MHz bw and the first used GSM channel.
If you want to read : 3GPP TR 25.816
Maybe I’m wrong, but I think the operator for which I’m working for at the moment is only using 200kHz guard band. This has to be confirmed.