Obviously it was brought from the north,I doubt any I1 is autochtonous here.
Here are my theories:
1)Slavs were brought by Goths in these areas,2 of 3 books for early Croatian history mentions Croats/Slavs in this are as 'Goths' or 'Goths,who are in fact Slavs','Croats alias Goths'
2)Goths arrived here just before Slavs and were assimilated,so in some way they are 'autochtonous' to this area,check this map:
http://www.s155239215.onlinehome.us/turkic/btn_GeographyMaps/AD_453WesternHunEmpire.gif
3)Croats may have assimilated great deal of I1 people in their old homeland.
Craniometrical measurements show that early Croats mostly correspond to skelets found on the border of north-eastern Germany and Poland;as I am aware,great deal of I1 is present there.If their 'homeland' was there,they surely assimilated a lot of I1.
4)People who came to western Balkans were simply Slavic-Gothic mix,an alliance of tribes.
5)Most extreme theory-Goths were Slavs themselves,parzitularly eastern Goths,some writers even claim that.