If you are staying in Gujarat POA can be registered in Gujarat and POA would be valid in Mumbai 

A GPA once duly registered with a Registrar of Assurances at one place is valid throughout India. There is no legal requirement for registering a GPA afresh in Mumbai for purchase of a property there. 

The registered POA, if executed authorizing you to sell the property located in Mumbai,  then you can proceed with the execution of sale deed in favor of the prospective buyer. 

the question is too generic and does not disclose proper details to enable to give an appropriate reply

if the POA is to be used for purpose of presenting any document for registration before the sub-registrar then if the grantor's address is at Gujarat, then this attested [or registered] POA will be valid for a Mumbai property also

I presume principal who executed PoA is resident of Gujarat as such he can execute registered POA in Gujarat for property situated in Mumbai 

Are the purposes - to sell, mortgage etc., restricted to properties in Gujarat? If so, the GPA cannot be used for purchasing a property in Mumbai. If not, it is very well valid.

- If the owner /principal of the property is residing in Gujrat , and unable to deal with the property which is located in Mumbai , can execute GPA on his behalf. 

- Hence, the said registered GPA is valid for the said work done which is mentioned in the said GPA. 

If the POA registered in Gujarat has mentioned the details of property situated in Mumbai, then the POA will be valid in Mumbai for the mentioned task.

It is important that the schedule of property is clearly mentioned in the POA deed.

If so then it is valid and need not register once again in Mumbai 

Dear client,

I would happily like to say you that a General Power of Attorney made in one State is valid in all the other States in India except for Jammu and Kashmir.

Hope this helps you.

Yes its valid 

Will be valid, as long as the person in whose favor it is given was residing in Gujarat when this poa was issued.