Asif bagged three wickets and Gul two as Sri Lanka reached 71-5 at lunch on the second day after being in to bat in overcast conditions. The first day's play was washed out on Sunday.
Tillakaratne Dilshan (33) and Farveez Maharoof (nine) were the not out batsmen at the break.
Asif struck in the first over of the match when he trapped opener Upul Tharanga (zero) leg-before with his fourth delivery and then bowled other left-hander Kumar Sangakkara (eight) with a ball that came in sharply.
Thilan Samaraweera (four) was bowled shouldering arms to Asif and with Gul striking twice, Sri Lanka fell to 32-5 and in threat of being bowled out for less than their lowest ever score of 71 against Pakistan at Kandy in 1994.
Gul bowled veteran opener Sanath Jayasuriya (six) and then Mahela Jayawardene fished outside the off stump to give him his second wicket.
Sri Lanka have lost captain Marvan Atapattu and Chaminda Vaas to injuries and Jayawardene is leading the side.
Pakistan are without middle-order batsman Mohammad Yousuf, who has been ruled out with a hamstring injury.
The second and final Test will be played in Kandy from April 3.
Pakistan won a preceding three-match one-day series 2-0 with one match abandoned due to rain.