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$ df -P
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/Rootvg-rootlv 20G 613M 19G 4% /
tmpfs 7.8G 0 7.8G 0% /dev/shm
/dev/sda1 240M 113M 115M 50% /boot
/dev/mapper/Rootvg-homelv 20G 2.0G 17G 11% /home
/dev/mapper/Rootvg-instlv 2.0G 2.9M 1.9G 1% /install
/dev/mapper/Rootvg-optlv 976M 53M 872M 6% /opt
/dev/mapper/Rootvg-itmlv 4.8G 10M 4.6G 1% /opt/IBM/ITM
/dev/mapper/Rootvg-tmplv 20G 2.8M 19G 1% /tmp
/dev/mapper/Rootvg-usrlv 32G 2.2G 28G 8% /usr
/dev/mapper/Rootvg-varlv 20G 198M 19G 2% /var
/dev/mapper/Rootvg-loglv 29G 73M 28G 1% /var/log
/dev/mapper/Rootvg-auditlv 27G 18M 26G 1% /var/log/audit
/dev/mapper/Appvg-applv 123G 868M 116G 1% /app

I am trying to a add the total Size (add up all shown in 2nd column)
How do I go by doing that ? as there are a combination of mega and giga

Use the -B option to specify the specific "blocksize" you want all values reported in. The blocksize is expressed with an integer and units. From the man page I have:

The SIZE argument is an integer and optional unit (example: 10K is
10*1024). Units are K,M,G,T,P,E,Z,Y (powers of 1024) or KB,MB,...
(powers of 1000).

For example, df -BM will print all the sizes in megabytes. (It will override a previous -h in case that is set in an alias; e.g., df -h -BM is that same)