Continuous Years

I may be misunderstanding what you are looking for but...

The total number of Fiscal years from 1970 to 2021 is 53 (2 FY years in 1970 & 2 FY years in 2021.

To calculate whether an ID is "Continuous", the count of the IDs FY must equal 53 (none do)

But anyway, here is how:

Create a numerical column for FY in your table using:

 To calculate the total number of FY:

Total FY Years =
VAR MinY =
    CALCULATE ( MIN ( 'Gifts (2)'[FY WN] ), ALL ( 'Gifts (2)' ) )
VAR MaxY =
    CALCULATE ( MAX ( 'Gifts (2)'[FY WN] ), ALL ( 'Gifts (2)' ) )
VAR FY_table =
    GENERATESERIES ( MinY, MaxY, 1 )
RETURN
    COUNTROWS ( FY_table )

To calculate if an ID is "Continuous":

Continuous Every year = 
IF(DISTINCTCOUNT('Gifts (2)'[FY WN]) = [Total FY Years], "Continuous")

For Lifetime Giving (non continous)

Lifetime Giving = DISTINCTCOUNT('Gifts (2)'[FY WN])

If you want to just use a single measure to combine both results

Combined =
IF (
    DISTINCTCOUNT ( 'Gifts (2)'[FY WN] ) = [Total FY Years],
    "Continuous",
    FORMAT ( [Life Giving], "##0" )
)

To include the first FY for each ID

Since =
"FY"
    & RIGHT (
        CALCULATE (
            MIN ( 'Gifts (2)'[FY WN] ),
            ALLEXCEPT ( 'Gifts (2)', 'Gifts (2)'[constituent_id] ),
            'Gifts (2)'[grouped]
        ),
        2
    )

To calaculate the number of FY without a donation

FY without donation =
VAR MinY =
    CALCULATE (
        MIN ( 'Gifts (2)'[FY WN] ),
        ALLEXCEPT ( 'Gifts (2)', 'Gifts (2)'[constituent_id] )
    )
VAR MaxY =
    CALCULATE (
        MAX ( 'Gifts (2)'[FY WN] ),
        ALLEXCEPT ( 'Gifts (2)', 'Gifts (2)'[constituent_id] )
    )
VAR FY_table =
    GENERATESERIES ( MinY, MaxY, 1 )
RETURN
    COUNTROWS ( FY_table ) - [Lifetime Giving]

I've attached the sample PBIX file