Replying to 310, the visitors had started the day on 262 for four and reached the interval on 339 for eight.
That gave them a lead of 29 with Katich unbeaten on 138.
Murray Goodwin fell 14 runs short of his century as Glamorgan trailed Hampshire by 43 runs on first innings in Cardiff.
The visitors had totalled 341 and Glamorgan went from an overnight 196 for three to 298 for six.
Calum Haggett claimed both wickets as Gloucestershire made 98 for two on the opening morning against Kent at Cheltenham.
Hagget removed Michael Klinger (20) and Dan Housego (13), with Chris Dent reaching lunch unbeaten on 53 for the home side.
- Sports & Recreation
- Murray Goodwin