How to stack values columns with different datatype into a single column [duplicate]

Question

I have a data set like this:

 dt <- tibble(
  TRIAL = c("A", "A", "A", "B", "B", "B", "C", "C", "C"),
  GYF = c(1, 2, 1, 1, 6, 3, 2, 3, 1),
  AD = c("A","B","C","D","E","F","G","H","I")
)     

# # A tibble: 9 x 3
#   TRIAL    GYF  AD
#   <chr> <dbl> <dbl>
# 1 A         1   A  
# 2 A         2   B
# 3 A         1   C  
# 4 B         1   D
# 5 B         6   E  
# 6 B         3   F
# 7 C         2   G
# 8 C         3   H
# 9 C         1   I 

This is what I want to achieve: I want all values of GYF and FY column to be stacked and having the Header and the Trial Value in the first column.

# # A tibble: 18 x 3
#   TRIAL  Trait Traitvalue
#   <chr> <chr> <dbl> 
# 1  A        GYF   1
# 2  A        GYF   2
# 3  A        GYF   3
# 4  A        AD    A
# 5  A        AD    B
# 6  A        AD    C
# 7  B        GYF   1
# 8  B        GYF   6
# 9  B        GYF   3
# 10 B        AD    D
# 11 B        AD    E
# 12 B        AD    F
# 13 C        GYF   2      
# 14 C        GYF   3
# 15 C        GYF   1
# 16 C        AD    G
# 17 C        AD    H
# 18 C        AD    I

When I run this code I get not the expected output

dt2 %>% pivot_longer(., c("GYF","AD")) %>% 
  arrange(., TRIAL, name) %>%
  rename(Traitvalue=value,
     Trait=name)

Gave me the Error( Error: Can't combine GYF and FY .) How can I possibly bypass the datatype issue to achieve my desired result. Thank you