I googled a bit and found videos sounding for me like [gallija], [gallia] and even [gallja], the first predominating. W. S. Allen's Vox Latina—a Guide to the Pronunciation of Classical Latin (page 40) says in a footnote that
The pronunciation [in some other situation] may in fact have been no different from that of words like diurnus, where the i would automatically induce a consonantal i-glide before another vowel,
but does not mention it in the main text. If [ij] is indeed correct, then why neither Wiktionary pronunciation guide nor other online sources I found mention [j]?