For the daily it should be correct, but I believe the GFS Monthly will be longer.

See this thread on the forum as there are some good explanations there - Immutability Duration and Retention - R&D Forums (veeam.com)

@dario72 If your GFS is for 1 Month only, Veeam will keep only 1 recent backup data file for you. For long-term policy, you should configure more than 1 month.

Also, you must be very careful of the repository capacity when using immutable. You cannot manually remove those immutable files via the Veeam manager console if it runs out of space.

Veeam provides a Veeam Backup Calculator tool you can use to estimate.

https://calculator.veeam.com/

I believe that the 1 month GFS retention is going to be redundant because you’re already keeping you’re incremental’s for 30 days.  GFS really shines when you’re keeping restore points for longer terms.  For instance, my standard setup is to keep a month of incremental’s, but then keep 11 monthly’s, and then 1, 3 or 7 yearly’s etc. 

Note that retention, for most part part, is separate from immutability as retention is set in the backup or copy job, but the immutability is set at the repository. 

For instance, if you want data to be immutable for 30 days, and your retention policy is for 60 days, then the data can be deleted starting at 30 days when the immutable flag expires but should still stick around until the retention policy takes effect.  On the converse, if you have a 30 day retention, but your immutability is set for 60 days, the files cannot be deleted until that immutability flag expires - in which case the files will be marked for deletion but will not be able to be deleted, but should be deleted once the flag expires as they were marked for deletion.