RCMP does not have access to GCMS. Access to GCMS is only shared between CBSA and CIC and maybe CSIS has some level of access to it but won't release any information as such.
With RCMP, yes, you can request any/all information related to arrest, detention, or whatever interaction you had with them which had to be documented. The only information related to immigration which mounties will provide is that your fingerprints were ran on CCRTIS and that they were destroyed afterwards. The latter is confirmed if you request it.
There is a lot of information which RCMP can choose not to disclose tho as they have a lot of immunity just like CSIS. The current ATIP processing time for RCMP is about 2-4 months and they won't notify you like CBSA if they need an extension from the standard 30 days. However, you can follow up after 30 days to check status by emailing atipb@rcmp- grc.gc.ca