http://www.utexas.edu/cola/centers/lrc/eieol/gotol-5-X.html#Got05_GP23

Strong verbs are characterized by vocalic alternation accompanying morphological change. This so-called ablaut pattern differs among verbs, but similarities are frequent enough that strong verbs fall into a small number of ablaut classes, the members of a given class sharing the same sequence of vocalic alternation. The particular sequence characterizing a given strong verb class is the result of a number of sound changes from Proto-Indo-European (PIE) to Proto-Germanic (PGmc), and from the latter to the respective daughter languages. The morpheme structure of the root in PIE uniquely characterizes the ablaut class to which a verb belongs; subsequent evolution into Germanic and the resulting daughter languages, however, has restructured this characterization. See this chart for an example of the evolution of a verb from PIE to the PGmc class I strong conjugation, and then gives examples of class I strong verbs from Gothic, and therefore WG.

Class I Ablaut Pattern

The difference between Classes Ia and Ib lies solely in the vowel of the past plural and participle stems. The change is conditioned by the consonant following the vowel. Generally the vowel i occurs, but this is replaced by ai when followed by h. Take for example the past participles stigans vs. þaihans.

The verb greipa 'seize', with prinicpal parts greipa -- gráip -- gripum -- gripans, serves to illustrate the forms of the first conjugation. The forms are as follows.