as discussed here.. https://www.maayboli.com/node/51711

itrans_gamabhana

1) Copy hindi itrans file to marathi gamabhana
a) sudo su
b) cd /usr/share/m17n/
c) cp hi-itrans.mim mr-gamabhana.mim

2) Open the file mr-gamabhana.mim and make the following 3 changes:
a) Change the following line to change the language from hindi to marathi
Old:
(input-method hi itrans)
New:
(input-method mr gamabhana)

b) Removed following dand signs as well as capital R, O and E:
  (".." "।")                          ; not in ITRANS Devanagari table
  (".," "॥")                          ; not in ITRANS Devanagari table
  (". " "।" (pushback 1))

c) Added following 5 entries:
I) in consonant section:
  ("R" "ऱ्")

II) in dependent section:
  ("O" (delete @-) "ॉ")    
  ("E" (delete @-) "ॅ")

III) in independent section:
  ("O" "ऑ")     
  ("E" "ॲ")
  
3) Optional change:

a) Independent
  ("ee" (set CANDRA-E-O 0) "ऎ")  
change "ऎ" to "ई" 
  ("ee" (set CANDRA-E-O 0) "ई")
  
  ("oo" (set CANDRA-E-O 0) "ऒ")
change "ऒ" to "ऊ"
  ("oo" (set CANDRA-E-O 0) "ऊ")

b) dependent
  ("ee" (delete @-) (set CANDRA-E-O 0) "ॆ") 
Change  "ॆ" to "ी"
  ("ee" (delete @-) (set CANDRA-E-O 0) "ी")
  
  ("oo" (delete @-) (set CANDRA-E-O 0) "ॊ")
change "ॊ" to "ू"
  ("oo" (delete @-) (set CANDRA-E-O 0) "ू")
  

Instructions to install gamabhana keyboard into ibus...

mkdir gamabhana
mkdir gamabhana/DEBIAN
mkdir -p gamabhana/usr/share/m17n/

wget https://raw.githubusercontent.com/shantanuo/spell_check/master/mr-gamabhana.mim -O /usr/share/m17n/mr-gamabhana.mim

cp /usr/share/m17n/mr-gamabhana.mim gamabhana/usr/share/m17n/

vi gamabhana/DEBIAN/control

Package: gamabhana
Version: 0.1
Maintainer: Shantanu Oak
Architecture: all
Description: gamabhana marathi keyboard for ibus

dpkg-deb --build gamabhana

dpkg -i gamabhana.deb