Do you have access to the oracle software owner account for the Grid Database? If so you can use the following to connect as sys to run the alter system command and connect to rman to run crosscheck and delete commands.
While logged in as the oracle software owner account for the Grid/OMS Database:
connect as sys:
$ sqlplus / as sysdba
SQL> alter system set db_recovery_file_dest_size=100G scope=both;
Connect to target database with RMAN catalog:
$ rman TARGET / CATALOG rman@RCAT
Connect to target database without RMAN catalog:
$ rman TARGET /
Then you can run your crosscheck and delete commands:
RMAN>
crosscheck backupset;
crosscheck copy;
crosscheck archivelog all;
delete noprompt obsolete;
delete noprompt expired backup;
delete noprompt expired copy;
If you don't have access to the oracle sofware owner account or sys privileges you'll need to obtain the access before you can make these changes.