How to get filter value of the recordType into a specific variable

See, these are the issues you face when you don't wanna use OOTB Export excel feature of Record based grids and instead try to duplicate the same functionality using Export DSE to Excel SS. There is no way you can save user filter values in any variable inside grid cause user filters and export are record functionalities.

The alternative here is to create custom filters which will look exactly like record user filters using combinations of cardLayout, sidebysideLayout and dopdownField and save the values in localVariables.

Now, since you can't use recordType as your data source in grid (due to custom filters) you will have to query the DSE or recordType whichever you prefer with the selected custom filters and pass the resultant datasubset to the data parameter in gridField and likely pass those filters in Export DSE to Excel SS too.

I will strongly recommend to go with the OOTB Export feature only until and unless there is some real cause for not using it.

See, these are the issues you face when you don't wanna use OOTB Export excel feature of Record based grids and instead try to duplicate the same functionality using Export DSE to Excel SS. There is no way you can save user filter values in any variable inside grid cause user filters and export are record functionalities.

The alternative here is to create custom filters which will look exactly like record user filters using combinations of cardLayout, sidebysideLayout and dopdownField and save the values in localVariables.

Now, since you can't use recordType as your data source in grid (due to custom filters) you will have to query the DSE or recordType whichever you prefer with the selected custom filters and pass the resultant datasubset to the data parameter in gridField and likely pass those filters in Export DSE to Excel SS too.

I will strongly recommend to go with the OOTB Export feature only until and unless there is some real cause for not using it.

Wait wait. You can still use record type as the data source when using custom filters. 
Refer to this - https://docs.appian.com/suite/help/23.1/fnc_system_recorddata.html

Either use the default export or The best of both worlds will be to use recordSource and use custom filters. 

Ah yes, we can use a!recordData directly in data parameter of gridField and apply custom filters.