You can create a VAT rate for this specific CGST and IGST. This way, they will show up, and you can select them upon making the bill. I'd be happy to walk you through on how to do that.
Let's starts by adding a new tax rate in the system. You'll need to create two tax rates in this case, one for CGST and one for IGST.
Here's how:
Click on Taxes at the left pane.
Hit on Add Tax.
In the Add Tax page, choose Tax rate.
Enter the details of your tax rate. If you need help with this screen, select the help button (yellow question mark).
Select Save when finished.
Repeat the same steps until you have the two different VAT rates already.
Once done, you can now generate a bill and utilize the said VAT rates. Make sure to use the rate per line item, so the IGST and CGST will be calculated in the bill.
Please see these sample snips for your visual reference: