If column equals then use value from another table

I have two tables with similar levels of data, and I want to run an IF formula to determin which table to pull the value from.

The two tables have a many-many relationship on Intersection (Although in my example it could be a many-one).

I effectively want the following formula, but I know that the logicaltest wont work against a column value, so any help to point in the right direction.

IF MEASURE = IF ( OR (ABS[Group], Forecast[GROUP]) = "GP1",  ABS[Value], Forecast[Value] )

Both ABS[Value] and Forecast[Value] have already been made into measures aswell.

Additionally, how to I make it so that the table has all the unique intersection values from both original tables.

Table 1 = Forecast

Table 2 = ABP

Expected Output Table

Google Drive Link to PBI Sample