EIRP is a power computation, it does not really matter which Technology (GSM, CDMA, UMTS, LTE). I’m saying this because now we have different frequency bands that can be used for UMTS and LTE.
If you are using purely active DAS there should be very slight difference between the EIRP of any Frequency Band (800,850,900, 1800, 1700, 1750, 2100, 2600), because the Technology will have the same Power at the Remote units, the slight difference will be due to a short jumper cable from the Remote unit to the Antenna.
EIRP = Source Power – System Losses + Atennna gain
Assuming you are using a Hybrid or a Purely Passive DAS, let us consider the equation above to answer your question.
Difference between GSM900 and I assume you mean UMTS2100, there will be no fix difference at all because of two Variables:
1. The system loss will be different depending on your product specification used in the DAS (Co-Axial Cable, jumper cables and the components Splitters and Couplers.
2. The Source Power, if you have different Power source value before feeding to the DAS then that should directly impact the EIRP of the two Technology on each Atennna.
In case you have the same power source then, the difference will be from the Product Specification, the loss in dB per Frequency Band of the Cable, Splitters and couplers used in the DAS.
Now, computation for RSSI can be considered fixed in an ideal environment between Frequency bands. For instance the difference between GSM900 and GSM1800 is equal to 6dB.
You can try different Frequency Band by using the Free Space Loss formula I have given on my first reply.
Do not forget to convert meters to Km before using the formula.
All the best,